How to Tell If Your Computer Has a Virus

If your computer is acting strangely — running slower than usual, showing unexpected pop-ups, or behaving in ways that do not quite add up — a virus or malware infection could be the cause. It is not always obvious at first. This guide covers the most common warning signs, how to confirm whether your computer is infected, and what to do next.

Common Signs That Your Computer May Have a Virus

Your computer has become noticeably slower

Malware often runs constantly in the background, consuming processor power and memory. If your computer has slowed down significantly and you have not installed anything new or had a major Windows update recently, it is worth investigating. Some slowdowns are explained by other causes — too many startup programs, a full hard drive, or an ageing machine — but a sudden change in performance is a warning sign.

You are seeing unexpected pop-ups or adverts

Pop-up adverts appearing outside of a browser, or adverts appearing on websites that do not normally show them, are a strong indicator of adware — a type of malware that injects advertising into your browsing experience. This is particularly common if you recently installed free software that bundled unwanted extras.

Your browser has changed without your permission

If your default search engine has changed, your homepage has been replaced, or there are browser extensions you did not install, your browser may have been hijacked. Browser hijackers redirect your searches through their own engines to generate advertising revenue.

Programs are crashing or behaving unexpectedly

Frequent application crashes, error messages on programs that usually work fine, or programs that seem to freeze and stop responding can all be symptoms of malware interfering with system processes.

Your hard drive is running constantly

If the hard drive activity light on your computer is running continuously even when you are not actively doing anything, something is running in the background. While Windows Update and antivirus scans are normal causes of this, sustained unexplained disk activity is worth checking.

You are being redirected to unfamiliar websites

If typing a web address takes you somewhere unexpected, or you are being redirected mid-browsing, malware may be intercepting your traffic.

Your antivirus has been disabled

Some malware specifically targets antivirus software, attempting to disable it so it cannot be detected. If Windows Defender or another security tool shows as disabled and you did not turn it off, treat this as a serious warning sign.

You are seeing ransom messages

If your screen shows a message claiming your files have been encrypted and demanding payment, this is ransomware. Do not pay. Read our guide on what to do if you get ransomware for the immediate steps to take.

How to Check for a Virus on Windows

Run a Windows Defender scan

1. Open Windows Security — search for it in the Start menu.
2. Click Virus and threat protection.
3. Click Quick scan to run an immediate check, or click Scan options to run a Full scan for a more thorough check.
4. Windows Defender will report any threats found and give you options to remove or quarantine them.

Run an offline scan for persistent malware

Some malware is designed to hide from antivirus tools that are running within Windows. Windows Defender includes an offline scan option that runs before Windows loads, making it harder for malware to hide.

1. Open Windows Security and click Virus and threat protection.
2. Click Scan options.
3. Select Microsoft Defender Antivirus (offline scan) and click Scan now.
4. Your computer will restart and scan before Windows loads.

Use Malwarebytes as a second opinion

Malwarebytes Free is a trusted tool for a second-opinion scan. Download it from malwarebytes.com, install it, and run a full scan. It is particularly good at catching adware and potentially unwanted programs that traditional antivirus tools sometimes miss.

What to Do If a Virus Is Found

If your antivirus detects something, follow its recommendations to quarantine or remove the threat. After removal, restart your computer and run another scan to confirm the threat has been fully cleared.

If you need help removing a specific infection, our guide on how to remove a virus from your PC covers the process in detail.

What If the Scans Come Back Clean But Problems Persist?

Not all computer problems are caused by viruses. If scans come back clean, the issue may be:

• Too many programs running at startup — check via Task Manager (Ctrl + Shift + Esc), then the Startup tab
• A full or near-full hard drive — Windows slows significantly when storage is very low
• Outdated drivers — particularly graphics drivers causing display issues
• Hardware failure — particularly a failing hard drive, which can cause crashes and slowdowns

For a broader set of speed fixes, read our guide on how to speed up Windows 11.