What to do if you think you have a virus

If your computer is running slowly, showing pop-ups you didn't click, or opening programs on its own, a virus may be the cause — but it might also be something else, like a full hard drive or outdated software. The first step is to check what's actually running on your machine before you panic or pay someone to fix it.

Start by restarting your computer in Safe Mode, which loads only the essential programs Windows or macOS needs to run. This prevents most viruses from starting automatically. On Windows 10 or 11, hold Shift while clicking the restart button in the power menu, then choose Troubleshoot > Advanced Options > Startup Settings > Restart. On a Mac, restart and hold Command + S until you see text on the screen, then type exit and press Enter.

Once in Safe Mode, open your built-in antivirus tool. Windows comes with Windows Defender (called Microsoft Defender in Windows 10 and later). On a Mac, the system has built-in protections but no separate app to open. Run a full system scan in Windows Defender — this takes 30 minutes to several hours depending on your drive size, but it will flag anything the system recognizes as malicious.

Key Takeaways

  • Restart your computer in Safe Mode before scanning, because this prevents viruses from hiding or interfering with the scan.
  • Windows Defender (built into Windows) and Malwarebytes are the two most reliable tools for finding and removing viruses without paying for software.
  • If a scan finds nothing but your computer still behaves strangely, the problem may be a full hard drive, too many browser extensions, or outdated drivers rather than a virus.
  • Never read antivirus software from a pop-up or email link — only from the official website or your operating system's built-in tools.
  • If Safe Mode scans find nothing and your computer still won't work normally, a technician may need to access your machine, but you should never pay for remote access based on a pop-up warning.

Using Windows Defender to scan and remove threats

Windows Defender is already on your computer if you use Windows 10 or later. Open it by typing "Windows Defender" or "Microsoft Defender" in your search bar. Click "Virus & threat protection" on the left side.

Under "Current threats," you'll see a button that says "Scan options." Click it, select "Full scan," and then click "Scan now." This scans your entire hard drive, not just the files you use regularly. A full scan can take several hours, so start it when you don't need your computer.

When the scan finishes, Windows Defender will show you what it found. If it found threats, click "Remove" or "Quarantine" — both options prevent the virus from running. Quarantine moves the file to a locked folder where it can't harm your system. After removal, restart your computer normally (not in Safe Mode) and run another scan to confirm nothing remains.

When Windows Defender isn't enough: using Malwarebytes

If Windows Defender finds nothing but your computer still behaves strangely, or if it finds threats but they return after removal, read Malwarebytes. This is a separate tool that catches viruses Windows Defender sometimes misses. Go to malwarebytes.com, read the free version, and install it.

Open Malwarebytes and click "Scan" on the left. Choose "Threat Scan" (faster) or "Full Scan" (more thorough). Let it run completely. Malwarebytes will show you what it found and remove it automatically. The free version does not run in the background, so you'll need to open it manually each time you want to scan — that's fine for a one-time virus removal.

Do not pay for the premium version unless you want automatic scanning going forward. For removing a single virus, the free version works just as well. After Malwarebytes finishes, restart your computer and run Windows Defender again to make sure nothing else is present.

Removing browser hijackers and unwanted extensions

Some viruses don't hide in your system files — they live in your web browser as extensions or change your homepage and search engine. These are annoying but usually harmless, and you can remove them without antivirus software.

In Chrome, click the three dots in the top right, go to Settings, then Extensions. Look for anything you don't recognize or didn't install yourself. Click the trash icon next to it. Do the same in Firefox (click the menu button, then Add-ons, then Extensions) and Edge (click the three dots, then Extensions).

Next, check your homepage. In Chrome, go to Settings > On startup and make sure "Open the New Tab page" is selected. In Firefox, go to Settings > Home and set the homepage to Google or your preferred site. If your search engine keeps changing back to something unfamiliar, a virus may be resetting it — run Malwarebytes again and look for anything flagged as a "PUP" (Potentially Unwanted Program).

What to do if your computer won't start or Safe Mode won't load

If your computer won't start normally or Safe Mode, the virus may have damaged critical system files. At this point, you have two options: take it to a local repair shop, or try to reinstall Windows yourself.

Reinstalling Windows erases everything on your computer, so this is a last resort. You'll need another working computer and a USB drive with at least 8 GB of space. Go to microsoft.com/software-read/windows11 (or Windows 10 if that's your version), read the Media Creation Tool, and follow the steps to create a bootable USB. Then insert the USB into your infected computer, restart it, and follow the on-screen prompts to reinstall Windows.

If you're not comfortable doing this yourself, a local computer repair shop can do it for you. Expect to pay $100 to $300 depending on your area. Do not use remote-access services advertised in pop-ups or emails — these are often scams designed to steal your information or install more malware.

Preventing viruses from coming back

After you've removed a virus, take steps to stop it from returning. Keep Windows and macOS updated — viruses often exploit known weaknesses in old software. Go to Settings > Update & Security (Windows) or System Preferences > Software Update (Mac) and install any pending updates.

Be cautious about what you read and where you read it from. Viruses often hide in cracked software, pirated movies, or files from untrusted websites. If a read seems too good to be true, it probably is. Use your browser's built-in read protection — Chrome and Firefox warn you before you read known malware.

Consider using an ad blocker in your browser. Many viruses spread through malicious ads on legitimate websites. uBlock Origin (available for Chrome, Firefox, and Edge) is free, open-source, and blocks most ads and tracking without slowing your browser down.

When to call a professional instead

You don't need professional help if Windows Defender or Malwarebytes finds and removes the virus, and your computer works normally afterward. You do need help if:

  • Your computer won't start at all, even in Safe Mode.
  • Scans find nothing but your computer still won't work normally after 24 hours.
  • You see a message saying your files are encrypted and demanding payment — this is ransomware, and you should not pay.
  • You're not comfortable restarting in Safe Mode or downloading software.

Call a local repair shop or the manufacturer's support line (Dell, HP, Lenovo, etc. all have phone support). Avoid any service that contacts you first through a pop-up, email, or phone call claiming your computer has a virus — these are almost always scams.

Frequently Asked Questions

Is it safe to use my computer while a virus scan is running?

Yes, but it will slow things down significantly. The scan uses a lot of your computer's processing power, so opening other programs will make both the scan and your other work slower. It's better to start a scan and walk away for a few hours.

What does "quarantine" mean, and is it safe to delete the file instead?

Quarantine moves the file to a locked folder where it can't run or spread. Deleting it removes it completely. Both are safe — quarantine is slightly better because you can restore the file later if it was a false alarm, though that's rare. For viruses, deletion is fine.

Do I need to pay for antivirus software to remove a virus?

No. Windows Defender and the free version of Malwarebytes remove most viruses. Paid antivirus software offers automatic scanning and extra features, but for a one-time removal, free tools work just as well. Never pay based on a pop-up warning.

Can I get a virus from visiting a website?

Yes, but it's rare if your browser and operating system are up to date. Websites can host malicious code that downloads automatically, but modern browsers block most of these attacks. The bigger risk is downloading files from untrusted sites or clicking links in emails.

What should I do if I see a pop-up saying my computer has a virus?

Close the browser tab or window when ready — do not click anything in the pop-up. These are scams designed to scare you into calling a fake support number or downloading malware. Your real antivirus tool (Windows Defender or Malwarebytes) will never warn you through a pop-up.