How to Know if Your PC Has a Virus: 9 Warning Signs

Modern malware is quieter than it used to be. Instead of locking your screen with a red banner, it hides in the background — slowing your PC, stealing credentials and sometimes encrypting files for ransom. Here are the nine signs we look for every week at the Repair Point workshop.

1. Your PC is suddenly slow — but only sometimes

Hidden miners and trojans use spare CPU cycles. If your laptop fan spins up while you're just reading email, open Task Manager (Ctrl+Shift+Esc) and look for an unfamiliar process pinning the CPU. A clean Windows 11 install should idle below 5%.

2. Pop-ups appear outside the browser

Browser pop-ups are normal. Pop-ups that appear when no browser is open — particularly ones offering antivirus, gift cards, or 'system warnings' — are almost always malware.

3. Your homepage or search engine has changed

Browser hijackers redirect your default search engine to ad-heavy sites. Resetting Chrome or Edge usually fixes the symptom but rarely removes the underlying extension.

4. Programs crash for no reason

Malware injects itself into legitimate processes. The result is random crashes in apps that worked yesterday — especially browsers, Outlook and Office.

5. You see new toolbars, extensions or shortcuts

Open Settings → Apps → Installed apps and Chrome → Extensions. Anything you don't remember installing should be removed (or investigated).

6. Files have weird extensions or won't open

If your photos suddenly end in .locked, .encrypted or .[random] — stop using the computer. This is a ransomware infection and continued use can destroy recoverable backups.

7. Your friends report strange messages from you

Email or social-media account takeovers often start on the device. If contacts say they got odd links from your address, change the password from a clean device first, then disinfect the PC.

8. Windows Defender has been turned off

Most malware disables Defender silently. Open Windows Security → Virus & threat protection. If real-time protection is off and you didn't disable it, treat the system as infected.

9. Your internet feels off

Unexpected DNS changes, blocked banking sites, or certificate warnings on sites that previously worked are all signs of network-level malware.

What to do next

1. Disconnect from Wi-Fi and Ethernet to stop data exfiltration.
2. Do not enter any banking or work passwords on the device.
3. Back up critical documents to an external drive only if you're sure they're not encrypted.
4. Run a full scan with Windows Defender Offline (Settings → Windows Security → Virus & threat protection → Scan options).
5. If anything is found — or symptoms persist — get a professional clean.