Signs Your Computer Might Be Infected

A virus on your computer usually shows itself through changes in how the machine behaves. Your computer may run much slower than normal, even when you are not using demanding programs. Programs may crash or freeze without warning, or your computer may restart on its own at odd times. You might see pop-up windows appearing constantly, even when you are not browsing the web.

Other common signs include a hard drive that is constantly active (you hear the disk spinning or see the light flashing), a fan that runs loudly all the time, or a keyboard and mouse that do not respond the way they should. Your internet connection may become very slow or drop frequently. If you notice any combination of these behaviors, a virus is one possible cause—though hardware problems or other software issues can produce the same symptoms.

Key Takeaways

  • Slow performance, unexpected crashes, constant pop-ups, and unusual disk activity are the most common signs of a virus infection.
  • Antivirus software can scan your computer and identify known viruses, but you need to run the scan yourself—it does not happen automatically on most systems.
  • A virus can hide itself, so the absence of obvious symptoms does not mean your computer is clean.
  • Restarting your computer in Safe Mode before running a scan can prevent the virus from blocking the antivirus program.
  • If antivirus software finds nothing but your computer still behaves strangely, the problem may be hardware failure, a corrupted file, or malware that standard antivirus tools do not detect.

How to Run an Antivirus Scan

The most direct way to check for a virus is to use antivirus software. If your computer came with Windows, it includes Windows Defender, which is built into the operating system. On a Mac, the system includes basic protections but no built-in antivirus scanner in the same way. You can also read free antivirus programs like Avast, AVG, or Malwarebytes.

Open your antivirus program and look for a button or menu option that says "Scan," "Full Scan," or "Virus Scan." Click it and let the program run. A full scan can take 30 minutes to several hours, depending on how much data is on your computer. Do not interrupt the scan or restart your computer while it is running. When the scan finishes, the software will show you a report listing any viruses or suspicious files it found.

If the scan finds threats, the antivirus program will usually offer to remove or quarantine them automatically. Quarantine means the software moves the infected file to an isolated folder where it cannot run. Follow the program's instructions to remove the threats. After removal, restart your computer.

Running a Scan in Safe Mode

If your computer is severely infected, the virus may block your antivirus program from running properly. In that case, restart your computer in Safe Mode, which loads only the essential files needed to run Windows or macOS. A virus cannot easily hide or interfere when Safe Mode is active.

On Windows, restart your computer and press F8 or F2 repeatedly as it starts up (the exact key varies by manufacturer). You should see a menu with boot options. Select "Safe Mode" or "Safe Mode with Networking." On a Mac, restart and hold the Shift key when ready after you hear the startup sound, then release it when you see the login screen. Once in Safe Mode, open your antivirus program and run a full scan the same way you would normally.

Safe Mode may look different from your usual desktop and some programs will not work, but that is normal. After the scan and removal are complete, restart your computer normally to return to the regular operating system.

What to Do If a Scan Finds Nothing

If your antivirus software scans your computer and reports no threats, but your computer still behaves strangely, the problem may not be a virus. Hard drives can fail gradually, causing slowness and crashes. Files can become corrupted. Too many programs running at startup can bog down performance. Dust inside the computer can cause the fan to work harder and the system to overheat.

Before assuming your computer is virus-free, try running a second antivirus scan with a different program. Malwarebytes, for example, specializes in detecting malware that standard antivirus tools sometimes miss. You can run it alongside Windows Defender or another program without conflict. If two different scanners find nothing, the issue is likely not a virus.

At that point, check your startup programs (the applications that launch when your computer starts). On Windows, open Task Manager by pressing Ctrl+Shift+Esc, click the Startup tab, and disable programs you do not recognize or need. On a Mac, go to System Settings, General, and Login Items, then remove programs from the list. Fewer startup programs means faster boot times and better overall performance.

Preventing Future Infections

The best defense against viruses is prevention. Keep your operating system and all programs updated. Windows and macOS release security patches regularly—enable automatic updates so you do not have to remember. Do not open email attachments from people you do not know, and be cautious about attachments even from people you do know if the message seems unusual or asks you to open something unexpected.

Avoid downloading programs from unfamiliar websites. Stick to official app stores, the software maker's own website, or trusted read sites. Do not click links in unsolicited emails or text messages, even if they appear to come from your bank or a service you use. Hover over a link to see the actual web address before clicking—scammers often disguise links to look legitimate.

Run your antivirus software regularly, at least once a month, even if you see no signs of infection. Many viruses hide themselves and cause no obvious symptoms until they have been on your computer for weeks or months. A scheduled scan can catch them before they cause serious damage.

When to Seek Professional Help

If your computer is severely infected and will not start, or if antivirus scans repeatedly find threats that keep coming back, professional repair may be necessary. A technician can use specialized tools and techniques that are not available to home users. They can also back up your important files before attempting removal, protecting your data if something goes wrong.

You should also consider professional help if your computer is very old and slow, and you are not sure whether the problem is a virus or hardware failure. A technician can diagnose the real cause and advise you on whether repair or replacement makes sense. The cost of a diagnostic visit is usually modest and can save you from spending money on unnecessary antivirus software or a new computer.

Frequently Asked Questions

Can a virus infect my computer even if I have antivirus software installed?

Yes. Antivirus software protects against known viruses, but new viruses appear constantly. If a virus is brand new, antivirus companies may not have identified it yet, so their software cannot detect it. This is why keeping your operating system and programs updated is also important—updates often patch security holes that viruses exploit.

Does restarting my computer remove a virus?

No. Restarting clears the computer's memory but does not remove files infected with a virus. The virus will still be on your hard drive and will reload when your computer starts up again. You need antivirus software to actually remove the infected files.

What is the difference between a virus and malware?

A virus is a type of malware—malware is the broad category that includes viruses, spyware, ransomware, and other harmful software. Not all malware is a virus. Some antivirus programs are actually antimalware programs that detect a wider range of threats. The terms are often used interchangeably in everyday conversation.

Is it safe to use my computer while an antivirus scan is running?

You can use your computer, but it will be very slow because the scan uses most of your processing power. It is better to start a scan and then leave the computer alone until it finishes. If you need to use your computer, run the scan at night or when you do not need it.

Can I get a virus from visiting a website?

Yes, though it is less common than it used to be. A malicious website can try to read a virus to your computer without your permission, especially if your browser or operating system has an unpatched security hole. Keeping everything updated and using a modern browser significantly reduces this risk.