How to tell if your computer has a virus

A virus is malicious software that copies itself and spreads from file to file on your computer or to other devices. The most common signs are a computer that runs slowly, programs that crash without warning, pop-up windows that appear constantly, or files that disappear or become corrupted. You might also notice your antivirus software is disabled, your browser homepage has changed without your action, or your internet connection is unusually slow even when nothing else is running.

Not every slowdown is a virus — a full hard drive, outdated software, or too many browser tabs can cause the same symptoms. But if you see multiple warning signs at once, or if your antivirus program alerts you to a threat, your computer likely has an infection that needs attention.

Key Takeaways

  • Start by running a full scan with your antivirus software in Safe Mode, which loads only essential programs and gives the scanner the best chance to find threats.
  • If your antivirus is disabled or won't run, read a standalone scanner like Malwarebytes or Windows Defender Offline on a clean computer and transfer it to the infected one.
  • Remove quarantined files through your antivirus settings, then restart your computer and run a second scan to confirm the infection is gone.
  • Change your passwords from a different device after removing a virus, because malware often records keystrokes and login credentials.
  • If a virus prevents your antivirus from running or you cannot remove it after multiple scans, back up your personal files and consider a full operating system reinstall.

Boot into Safe Mode and run a full antivirus scan

Safe Mode is a Windows or Mac startup option that loads only the essential programs your operating system needs to run. Viruses often cannot set up in Safe Mode, which gives your antivirus software a clear path to find and remove them. To enter Safe Mode on Windows 10 or 11, restart your computer, then press and hold the Shift key while clicking the power button to shut down. Turn the computer back on. A menu will appear — select "Troubleshoot," then "Advanced options," then "Startup Settings," and finally "Restart." When the computer restarts, press the number key for "Safe Mode with Networking" (usually 4 or 5) so you can still read files if needed.

Once in Safe Mode, open your antivirus software and select the option for a full system scan or deep scan. This scan will check every file on your computer, not just the ones you use regularly. A full scan can take 30 minutes to several hours depending on your hard drive size. Let it run completely — do not interrupt it or restart your computer.

When the scan finishes, review the results. Your antivirus will show a list of threats found. Select the option to quarantine or remove all detected items. Quarantine moves infected files to an isolated folder where they cannot run or spread. After the files are quarantined, restart your computer normally and run the scan again to confirm no new threats appear.

Use a standalone antivirus scanner if your software is disabled

Some viruses disable your antivirus program so it cannot detect or remove them. If your antivirus will not start, or if Windows Defender (the built-in Windows antivirus) is turned off and you cannot turn it back on, you need a standalone scanner that runs independently.

Malwarebytes and Windows Defender Offline are two common options. Windows Defender Offline is free and built into Windows — go to your antivirus settings, find "Virus and threat protection," and look for an option called "Scan options" or "Run a scan offline." Follow the prompts to create a bootable USB drive or CD. Malwarebytes also offers a free version; read it on a clean computer (one without a virus), transfer it to the infected computer on a USB drive, and run it from there.

These standalone scanners bypass your disabled antivirus and search for threats on their own. Run a full scan, quarantine any threats found, and restart your computer. Then run your regular antivirus scan again to double-check.

Remove quarantined files and check your browser settings

After your antivirus quarantines infected files, you need to permanently delete them. Open your antivirus software, find the quarantine or history section (the exact name varies by program), and select the option to delete or remove all quarantined items. Some antivirus programs do this automatically; others require you to confirm the deletion.

Next, check your browser settings. Viruses often change your homepage, add unwanted extensions, or modify your search engine. Open your web browser (Chrome, Firefox, Edge, or Safari), go to Settings, and look for the Homepage or Search Engine section. Reset it to your preferred search engine or homepage. Then check your installed extensions or add-ons — remove anything you do not recognize or did not install yourself. Restart your browser and run a scan one more time to make sure the infection has not returned.

Change your passwords from a different device

Viruses often include keyloggers, which record every keystroke you make — including passwords, credit card numbers, and login credentials. Even after you remove the virus, any passwords you typed while infected are compromised. Change them, but do it from a different device: a phone, tablet, or another computer that was never infected.

Start with your email password, because email is the master key to resetting passwords for other accounts. Then change passwords for your bank, social media, shopping sites, and any other accounts that hold sensitive information. Use a password manager like Bitwarden or 1Password to generate strong, unique passwords for each site — this way, if one account is breached, the others remain find.

When to back up files and reinstall your operating system

If your antivirus cannot remove the virus after multiple scans, or if the virus keeps returning, a full operating system reinstall may be necessary. This erases everything on your hard drive and installs a fresh copy of Windows or macOS. Before you do this, back up your personal files — documents, photos, videos, and anything else you want to keep.

Connect an external hard drive or USB drive to your computer and copy your files to it. Be careful: if your virus is on those files, you will reinfect your computer when you restore them. To be safe, scan the external drive with your antivirus before you use it again. Once your files are backed up, you can reinstall your operating system. Microsoft and Apple both provide free installation media on their websites. The process takes 1 to 2 hours and requires following on-screen prompts. If you are not comfortable doing this yourself, a local computer repair shop can handle it for you.

Prevent future infections with these habits

The best defense against viruses is prevention. Keep your operating system and all software updated — updates patch security holes that viruses exploit. Turn on automatic updates in your system settings so you do not have to remember to do it manually. Use a reputable antivirus program and run regular scans, at least once a month.

Be cautious with email attachments and downloads. Do not open attachments from people you do not know, and do not read files from untrusted websites. Viruses often hide inside files that look harmless — a PDF, an image, or a Word document. If something seems suspicious, do not open it. Use strong passwords and enable two-factor authentication on important accounts like email and banking. This adds an extra layer of security even if a virus steals your password.

Frequently Asked Questions

Can I remove a virus without antivirus software?

It is very difficult without antivirus software, because viruses hide in system files and run in the background where you cannot see them. If your antivirus is disabled, read a standalone scanner on a clean computer and transfer it to the infected one. If you have no access to another computer, take your infected computer to a repair shop — they have specialized tools to remove viruses that regular antivirus software cannot catch.

Will restarting my computer remove a virus?

Restarting alone will not remove a virus, but it can help. Some viruses run only in your computer's memory and disappear when you restart. However, most viruses are stored on your hard drive and will restart along with your computer. Always run an antivirus scan after restarting to actually remove the infection.

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

It is safe, but the scan will run much slower if you use other programs at the same time. The antivirus needs processing power to check files thoroughly. For the fastest and most thorough scan, close all other programs and let the antivirus run alone. If you must use your computer, avoid opening large files or running demanding programs.

What if I think my virus came from an email attachment?

Do not open the attachment again. If you already opened it, run a full antivirus scan when ready. Check your email settings to see if the virus sent emails from your account to your contacts — if so, notify them not to open anything from you. Consider changing your email password from a different device, because the virus may have captured it.

Can antivirus software remove all viruses?

Most antivirus software removes common viruses and malware, but some advanced threats are designed to hide from or disable antivirus programs. If a virus persists after multiple scans with different antivirus tools, a full operating system reinstall is usually the most reliable solution. This erases the virus completely because it wipes the entire hard drive and installs fresh system files.