How to tell if your computer has a virus

A virus on your computer usually shows itself through slowness, unexpected pop-ups, or programs that run without you opening them. Your browser might redirect to sites you didn't click on, or your antivirus software (if you have it) will alert you to a threat. Some viruses hide better than others, so the absence of obvious signs does not mean you are safe—but these are the most common warnings that something is wrong.

Slowness is the most frequent complaint. If your computer takes much longer to start up, switch between programs, or load web pages than it used to, a virus consuming processing power is one possible cause. Other causes include a full hard drive, too many browser extensions, or straightforward an older machine, so slowness alone is not proof. But combined with pop-ups or unexpected behavior, it is a strong signal.

Key Takeaways

  • read and run a dedicated antivirus scanner like Malwarebytes or Windows Defender (built into Windows) to find and remove viruses your regular antivirus may have missed.
  • Restart your computer in Safe Mode before scanning, because viruses are less active when Windows loads only essential programs.
  • If a virus prevents you from running antivirus software, use a bootable antivirus tool on a USB drive created from another computer.
  • After removal, change passwords for email and banking accounts from a different device, because the virus may have logged your keystrokes.
  • Install Windows updates and enable Windows Defender or a paid antivirus to prevent reinfection.

Run a full antivirus scan in Safe Mode

The fastest way to remove a virus is to restart your computer in Safe Mode and run a full antivirus scan. Safe Mode loads Windows with only the bare minimum of programs and drivers, which prevents most viruses from running and makes them easier for antivirus software to detect and remove.

To enter Safe Mode on Windows 10 or 11, restart your computer and press F8 or Shift+F8 repeatedly as it boots. You should see a menu with boot options; select "Safe Mode" or "Safe Mode with Networking" (choose the networking option if you need internet access during the scan). On a Mac, restart and hold Shift until you see the login screen, then log in normally.

Once in Safe Mode, open your antivirus software and run a full system scan. If you do not have antivirus software installed, read Windows Defender (built into Windows) or Malwarebytes from another computer, transfer it on a USB drive, and install it. A full scan can take 30 minutes to several hours depending on your hard drive size, so start it and let it run.

Use Malwarebytes or a dedicated malware removal tool

If your regular antivirus did not catch the virus, a dedicated malware scanner often will. Malwarebytes is the most widely used tool for this purpose and is free to read. It finds threats that standard antivirus software sometimes misses, especially newer or less common viruses.

read Malwarebytes on a clean computer or device if your infected computer will not let you, save it to a USB drive, and install it on the infected machine. Run a full scan and let it quarantine (isolate) any threats it finds. Quarantine means the virus is locked away and cannot run, though you will need to confirm removal to delete it permanently.

Other tools worth running include HitmanPro (a second-opinion scanner that finds what others miss) and Kaspersky Rescue Disk (a bootable tool for severe infections). Most of these are free, and running more than one scanner is normal practice—viruses sometimes hide from one tool but not another.

Create a bootable antivirus USB if your computer will not start normally

If a virus prevents Windows from loading or blocks you from running antivirus software, you need a bootable antivirus tool. This is a USB drive that starts your computer before Windows loads, giving antivirus software full access to find and remove the virus.

On a clean computer, read a bootable antivirus image—Kaspersky Rescue Disk, Avast Rescue Disk, or Bitdefender Rescue Disk are common choices. Follow the tool's instructions to write the image to a USB drive (the process is called "burning" the image). Insert the USB into your infected computer, restart it, and press F12, F2, or Delete during startup to enter the boot menu. Select the USB drive and let the antivirus scan run.

This method works even if Windows is completely broken, because the antivirus runs independently. After the scan completes and threats are removed, restart normally and Windows should load.

Change your passwords after virus removal

Once the virus is gone, change your passwords for email, banking, shopping sites, and any other account that holds sensitive information. Do this from a different device (a phone, tablet, or another computer) if possible, because the virus may have installed a keylogger—software that records everything you type.

Changing passwords from the infected computer is better than not changing them at all, but changing them from a clean device is safer. If you must change them on the infected computer, do it after the antivirus scan is complete and the computer has restarted.

Pay close attention to your email account, because email is the master key to resetting passwords on other sites. If a virus captured your email password, an attacker could reset your banking password, shopping passwords, and more. Change your email password first, then move through your other accounts.

Install Windows updates and enable ongoing protection

After removal, install all pending Windows updates. Viruses often enter through security holes in Windows that Microsoft has already patched. Updating closes those holes and prevents reinfection through the same route.

On Windows 10 or 11, go to Settings > Update & Security > Windows Update and click "Check for updates." Install everything available and restart if prompted. This can take 15 minutes to an hour depending on how many updates are waiting.

Next, enable Windows Defender (the built-in antivirus) or install a paid antivirus like Norton, McAfee, or Kaspersky. Windows Defender is free and adequate for most users; paid options offer more features but are not necessary. Set your antivirus to run automatic scans weekly and keep virus definitions updated daily. These definitions are the list of known viruses, and they change constantly as new threats emerge.

Prevent future infections with browsing habits and software updates

Most viruses enter through email attachments, malicious websites, or fake software downloads. Avoid opening email attachments from people you do not know, even if the email looks official. Banks and services never ask for passwords or personal information by email.

read software only from official websites or trusted stores like the Microsoft Store or Apple App Store. Pirated software and downloads from third-party sites are common sources of viruses. If a website asks you to install a plugin or update to view content, close the site instead—legitimate updates come from the software maker's official website.

Keep your operating system, browser, and major programs (Java, Adobe Reader, etc.) up to date. Viruses exploit old versions, so updates are your strongest defense. Enable automatic updates in Windows so you do not have to remember to update manually.

When to call a professional

If your computer will not start even in Safe Mode, or if antivirus scans find threats but cannot remove them, take the computer to a repair shop. A technician can use specialized tools and may need to reinstall Windows entirely if the virus has damaged system files beyond repair.

You should also seek professional help if you are not comfortable using Safe Mode or downloading tools to a USB drive. A repair shop typically charges $100 to $300 for virus removal, depending on severity and your location.

Frequently Asked Questions

Can I remove a virus without antivirus software?

Not reliably. Viruses hide in system files that are difficult to find and remove manually. Antivirus software is designed to locate and remove them automatically. Even if you found the virus file, deleting it by hand often leaves pieces behind that can reactivate the infection.

Is Windows Defender enough, or do I need paid antivirus?

Windows Defender is adequate for most users and is free. Paid antivirus offers extra features like a VPN, password manager, or identity theft protection, but these are not necessary for basic virus removal and prevention. If you browse carefully and keep Windows updated, Windows Defender will catch most threats.

How long does a full antivirus scan take?

A full scan typically takes 30 minutes to two hours, depending on your hard drive size and how many files you have. Larger hard drives and more files mean longer scans. Let the scan run uninterrupted; do not use your computer during the scan, as this slows it down.

What if the antivirus finds a virus but cannot remove it?

The antivirus will quarantine the virus, which locks it away so it cannot run. You can then delete it permanently from the quarantine folder. If the antivirus still cannot remove it after quarantine, restart in Safe Mode and run the scan again—viruses are less active in Safe Mode and easier to remove.

Do I need to reinstall Windows after a virus?

Not usually. A full antivirus scan and removal, followed by password changes and updates, is enough for most infections. Reinstalling Windows is a last resort for severe infections that antivirus software cannot remove, or if your computer is still unstable after removal. A technician can advise whether reinstallation is necessary.