Is your computer suddenly running slowly, showing strange pop-ups, redirecting your browser, or behaving in ways you don’t recognize? These changes can sometimes be signs that malware has found its way onto your device.

Malware—short for malicious software—can affect computers in many different ways. Some malware is designed to steal information, while other forms may display unwanted advertisements, interfere with normal operation, or make a device vulnerable to additional threats. The Federal Trade Commission (FTC) specifically identifies unexpected slowdowns, crashes, browser redirects, new toolbars, excessive pop-ups, disabled system tools, and unauthorized messages as possible warning signs.

However, an important point is often overlooked: one unusual computer problem doesn’t automatically mean you have malware. Slow performance, crashes, and browser problems can also have ordinary technical causes.

In this guide, you’ll learn the 15 most common signs your computer may have malware, how to distinguish suspicious behavior from normal computer problems, and what you should do if you suspect an infection.

What Is Malware?

Malware is software designed to perform harmful or unwanted actions on a computer or other device without the user’s knowledge or permission.

Common types include:

Viruses — malicious programs that can spread by infecting other files or systems.
Spyware — software designed to monitor activity or collect information.
Ransomware — malware that can make files or systems inaccessible and demand payment.
Trojans — malicious software disguised as something legitimate.
Adware — software associated with unwanted advertisements.
Worms — malware capable of spreading between systems.

Depending on its type, malware can steal personal information, interfere with normal computer operation, display unwanted advertisements, or create additional security risks.

That’s why recognizing unusual behavior early is important.

15 Signs Your Computer May Have Malware

1. Your Computer Suddenly Becomes Very Slow

One of the most noticeable signs of a potential malware infection is a sudden and unexplained slowdown.

You may notice that:

Programs take much longer to open.
Websites load slowly.
The computer takes longer to start.
Even simple tasks feel unusually sluggish.
Switching between applications becomes difficult.

Malware can consume system resources in the background, potentially affecting normal performance. The FTC lists computers becoming slow, freezing, or crashing among possible malware symptoms.

However, don’t immediately assume malware is responsible. Low storage, too many startup applications, outdated hardware, software updates, or legitimate background processes can also cause slow performance.

2. Your Computer Frequently Freezes or Crashes

Frequent freezing or unexpected crashes can be another warning sign.

If your computer was previously stable but suddenly begins:

Freezing regularly
Closing applications unexpectedly
Showing repeated system errors
Restarting unexpectedly
Becoming unresponsive

it’s worth investigating.

The FTC includes freezing and crashing among symptoms that could indicate malware.

Still, hardware problems, incompatible software, overheating, and operating-system issues can produce similar symptoms. Look for other warning signs before concluding that malware is the cause.

3. You See Lots of Unexpected Pop-Ups

Suddenly seeing excessive pop-ups—especially when you’re not visiting websites that normally display them—can be a red flag.

Suspicious pop-ups might claim:

Your computer has a virus.
Your security subscription has expired.
You need to call technical support.
You need to download a security application immediately.
You have won a prize.

Be particularly careful with pop-ups that tell you to call a phone number. The FTC warns that fake security warnings are commonly used in tech-support scams.

Never assume a pop-up is legitimate simply because it uses the logo of a well-known technology company.

If you’re concerned about your computer, open your trusted security software yourself and run a scan rather than following instructions inside a suspicious pop-up.

4. Your Browser Keeps Redirecting You

Does clicking one website unexpectedly take you somewhere completely different?

Repeated browser redirects can be a warning sign, especially when they happen without an obvious explanation.

You might notice:

Search results leading to unfamiliar websites.
Unexpected pages opening.
New tabs appearing automatically.
Links taking you somewhere other than expected.

Browser redirects can sometimes be associated with unwanted software or browser manipulation.

However, don’t assume every redirect is malware. Websites, browser extensions, advertisements, and incorrect settings can also cause redirects.

5. Your Homepage or Search Engine Changes Without Permission

Another potential warning sign is discovering that your browser homepage or default search engine has changed without you intentionally changing it.

For example, you might open your browser and discover:

An unfamiliar homepage
A different search engine
Unexpected new-tab behavior
Strange search results

The FTC lists unexplained homepage changes and automatic redirects among possible malware symptoms.

Check whether the change occurred after installing legitimate software or a browser extension. If you cannot explain it, investigate further.

6. Unknown Browser Extensions or Toolbars Appear

Have you suddenly noticed a browser extension, add-on, or toolbar that you don’t remember installing?

This deserves attention.

Look for:

Extensions you don’t recognize
Toolbars that appeared unexpectedly
Extensions with suspicious names
Extensions you cannot remember approving

The FTC specifically identifies new browser toolbars and add-on programs as possible signs of malware.

Before removing something, make sure it isn’t a legitimate extension installed by you or another authorized user.

7. Unknown Programs Appear on Your Computer

Unexpected applications or desktop shortcuts can also be a warning sign.

You might discover software that you don’t remember downloading or installing.

Ask yourself:

Did someone else use this computer?
Was new software recently installed?
Did a legitimate application install additional components?
Did the software appear after downloading something from an unfamiliar source?

Don’t delete random system files simply because their names look unfamiliar. Some legitimate operating-system components have technical names that ordinary users may not recognize.

Instead, investigate unfamiliar applications carefully using trusted security tools or qualified technical support.

8. Your CPU, Memory, or Disk Usage Is Unusually High

A computer can become slow when applications consume significant CPU, memory, or disk resources.

Potential warning signs include:

Fans running frequently
High CPU usage while you’re doing very little
Heavy disk activity
Applications becoming sluggish
The computer getting unusually warm

Malware can sometimes run processes in the background, but high resource usage has many legitimate explanations.

For example, operating-system updates, video editing, cloud synchronization, browser tabs, and security scans can temporarily consume substantial resources.

The key is to investigate unexplained and persistent activity.

9. Your Internet Connection Suddenly Seems Unusually Slow

If one computer suddenly uses much more network activity than expected, malware could potentially be one possible explanation.

You may notice:

Websites loading unusually slowly
Downloads behaving differently
Unexpected network activity
Internet problems occurring mainly on one computer

But slow internet doesn’t automatically indicate malware.

Your router, internet service provider, Wi-Fi signal, network congestion, or multiple connected devices could be responsible.

Compare the experience across other devices before reaching a conclusion.

10. Security Software or System Tools Stop Working

This is one of the more concerning symptoms.

You may find that security software unexpectedly stops functioning or system tools behave differently.

Examples can include:

Security software being disabled unexpectedly
Task Manager or similar system tools becoming unavailable
Security settings changing without your knowledge
Repeated problems opening security applications

The FTC specifically lists disabled operating-system tools such as Task Manager or Activity Monitor as a possible malware symptom.

If this happens together with several other warning signs, consider running a trusted security scan and seeking qualified technical help.

11. You Keep Seeing Strange Error Messages

Unexpected or repeated error messages can indicate a software problem—and sometimes malware.

Be cautious if you suddenly see:

Repeated unexplained errors
Strange security warnings
Messages you’ve never seen before
Alerts asking you to download unknown software

Importantly, not every security warning is fake. Modern operating systems and browsers regularly display legitimate alerts.

The safest approach is to verify the warning through the relevant application’s official settings or website rather than clicking unfamiliar buttons inside a suspicious message.

12. Emails or Social Media Messages Are Sent Without You

Imagine a friend tells you that they received a strange message from your account, but you never sent it.

That deserves investigation.

The FTC lists emails you didn’t write and social-media messages you didn’t post among possible malware symptoms.

However, unauthorized messages don’t necessarily prove malware. Your online account itself may have been compromised.

If you notice this behavior, secure the affected account and review its recent activity. If you suspect the computer itself is compromised, avoid entering additional sensitive information on it until you’ve investigated the problem.

13. Your Files Suddenly Change, Disappear, or Become Inaccessible

Unexpected file changes deserve immediate attention.

Warning signs may include:

Files becoming inaccessible
Documents changing unexpectedly
Unfamiliar file extensions
Important files suddenly disappearing
Unexpected messages demanding payment to restore access

Some forms of malware, particularly ransomware, can interfere with access to files.

If important files suddenly become inaccessible, avoid experimenting with random “recovery” software or following suspicious instructions. Instead, use trusted security guidance and consider professional assistance.

Regular backups are one of the most important ways to reduce the impact of serious malware incidents.

14. Strange Behavior Starts After a Suspicious Download or Link

Timing matters.

If your computer begins behaving strangely shortly after you:

Downloaded software from an unfamiliar website
Opened an unexpected attachment
Clicked a suspicious link
Installed an unofficial application
Followed an unexpected security prompt

it’s worth investigating.

The FTC warns that deceptive phishing messages can trick users into clicking links or opening attachments that download malware.

Recent FTC guidance has also warned about fake CAPTCHA pages that attempt to trick people into running commands that can install malware.

The safest rule is simple: don’t run unfamiliar commands or install software because a random webpage tells you to.

15. Several Warning Signs Appear at the Same Time

One symptom isn’t necessarily enough to conclude that malware is present.

But multiple unexplained symptoms occurring together deserve more attention.

For example:

Sudden slowdown + browser redirects + unknown extensions + excessive pop-ups

is more concerning than a computer simply becoming slow after a major software update.

Think in terms of patterns rather than individual symptoms.

Are These Signs Always Caused by Malware?

No.

This is one of the most important things to understand.

Many common computer problems can look like malware symptoms. For example:

Low storage can make a computer slow.
Old hardware can cause poor performance.
Browser extensions can change search behavior.
Software bugs can cause crashes.
Operating-system updates can temporarily increase resource usage.
Network problems can make websites load slowly.
Legitimate notifications can look unfamiliar.

Therefore, symptoms alone cannot conclusively diagnose malware.

The goal is to recognize unusual patterns and then investigate them using trusted security tools.

How to Check If Your Computer Has Malware

If you suspect your computer may have malware, don’t panic. Follow a cautious approach.

1. Stop entering sensitive information

If you strongly suspect an infection, avoid using the computer for banking, shopping, or entering important passwords until you’ve investigated the issue.

The FTC recommends stopping sensitive online activity when malware is suspected.

2. Update your security software

Make sure your trusted security software is current.

Security tools need current protection information to identify newer threats.

3. Run a security scan

Use your reputable security software to perform an appropriate scan.

Follow the software’s recommendations if it identifies suspicious files or applications.

4. Review unfamiliar software and browser extensions

Look for applications or extensions you don’t recognize.

Don’t remove important system components simply because they’re unfamiliar. If you’re unsure, seek qualified assistance.

5. Update your operating system and applications

Keeping software updated is a fundamental security practice. CISA guidance emphasizes timely patching and current endpoint protections.

What to Do If You Find Malware

If a security scan identifies malware, take the situation seriously.

A sensible response includes:

Follow your security software’s removal or quarantine recommendations.
Avoid entering sensitive information on the affected device.
Update your security software and operating system.
Run another scan if recommended.
Change important passwords from a trusted device if account compromise is possible.
Enable multi-factor authentication where available.
Restore important files from a known-good backup when appropriate.
Seek help from trusted technical support if you cannot resolve the problem.

The FTC recommends updating security software, running a scan, changing passwords, and enabling two-factor authentication when malware may have exposed accounts.

How Does Malware Get on a Computer?

Understanding common infection routes can help you prevent future problems.

Malware may be delivered through:

Phishing emails
Suspicious links
Unexpected attachments
Untrusted downloads
Fake software updates
Malicious advertisements
Compromised websites
Unfamiliar file-sharing sources

The FTC recommends avoiding unfamiliar download websites and unexpected links or attachments, while CISA emphasizes current software and endpoint protection.

One useful rule is:

If you didn’t expect it, don’t click it immediately.

Take a moment to verify what you’re being asked to open or download.

How to Prevent Malware Infections

You don’t need to be a cybersecurity expert to improve your protection.

Follow these basic habits:

Keep your operating system updated.
Update your browser and applications.
Use reputable security software.
Don’t click unexpected links.
Be cautious with email attachments.
Download software from trusted sources.
Avoid suspicious free-download websites.
Keep important files backed up.
Use strong, unique passwords.
Enable multi-factor authentication.

CISA recommends timely software updates, endpoint protection, MFA, and backups as important security measures.

Malware vs. Virus: What’s the Difference?

The terms “virus” and “malware” are often used interchangeably, but they’re not exactly the same.

Term	Meaning
Malware	Broad term for malicious software
Virus	A type of malware that can replicate by infecting files
Spyware	Malware associated with monitoring or collecting information
Ransomware	Malware that can make data inaccessible and demand payment
Trojan	Malicious software disguised as legitimate software
Adware	Software associated with unwanted advertisements

Understanding this distinction makes it easier to search for the right security information when something goes wrong.

When Should You Get Professional Help?

Consider getting help from a trusted technical professional if:

Malware cannot be removed.
Security tools continue to malfunction.
Important files are inaccessible.
The computer repeatedly shows suspicious behavior.
You suspect important accounts have been compromised.
You don’t feel confident troubleshooting the issue yourself.

The FTC recommends contacting your device manufacturer or a trusted technical-support provider if you’re unable to resolve a malware problem yourself.

Avoid anyone who unexpectedly contacts you claiming that they detected a virus and immediately asks for payment, remote access, or sensitive information. Fake tech-support warnings are a known scam pattern.

Quick Malware Warning Signs Checklist

Save or share this checklist for future reference:

☐ Computer suddenly becomes slow
☐ Frequent freezing or crashes
☐ Excessive unexpected pop-ups
☐ Browser redirects
☐ Homepage changes without permission
☐ Unknown browser extensions
☐ Unfamiliar programs appear
☐ Unusually high CPU or disk activity
☐ Unexpected network activity
☐ Security tools stop working
☐ Strange error messages
☐ Emails or messages are sent without you
☐ Files unexpectedly change or become inaccessible
☐ Strange behavior follows a suspicious download
☐ Several symptoms appear at the same time

Frequently Asked Questions About Malware Symptoms

What are the first signs of malware?

Common warning signs can include sudden slowdowns, freezes, crashes, excessive pop-ups, browser redirects, unexpected browser changes, and unfamiliar software. However, these symptoms can have other causes too.

How do I know if my computer has malware?

Symptoms alone can’t confirm an infection. If you notice multiple unexplained warning signs, use reputable security software to run a scan and investigate unfamiliar applications or browser extensions.

Can malware make a computer slow?

Yes. Malware can potentially consume system resources and affect normal performance. However, low storage, old hardware, software conflicts, and background processes can also make a computer slow.

Can malware cause pop-ups?

Yes. Excessive or unexpected pop-ups can be associated with malware or unwanted software. But some legitimate websites and browser notifications can also produce pop-ups.

Can malware steal passwords?

Some types of malware can target account credentials and other sensitive information. That’s why suspected infections should be taken seriously, particularly if the computer is used for important accounts.

Can antivirus software detect every type of malware?

Security software is an important layer of protection, but no single security measure should be treated as perfect. Keep security tools updated and combine them with safe browsing, software updates, backups, and good account-security practices.

What should I do if I think my computer has malware?

Stop entering sensitive information on the potentially affected device, update trusted security software, run a scan, follow its recommendations, and secure important accounts if necessary. If you can’t resolve the problem, contact trusted technical support.

Final Takeaway: Don’t Ignore Unusual Computer Behavior

Malware doesn’t always announce itself with an obvious warning. Sometimes the first clues are subtle: a computer that suddenly becomes slow, a browser that starts redirecting, unfamiliar extensions, strange pop-ups, or unexpected account activity.

The good news is that recognizing these warning signs gives you an opportunity to respond before a potential problem becomes more serious.

Remember: one symptom doesn’t automatically mean your computer has malware. Look for unusual patterns, verify suspicious behavior with trusted security tools, keep your software updated, maintain reliable backups, and avoid unexpected links and downloads.

If you found this guide useful, bookmark it and share the malware warning-sign checklist with friends, family, classmates, or coworkers. Knowing what to look for is one of the simplest ways to improve everyday cybersecurity.