7 signs your computer might be infected with malware (and what to do)

Published August 27, 2026 · 6 min read

Edson Feitosa de Oliveira Junior Founder & Lead Engineer · PruPru Digital Services

Not every slow computer is infected, but a few specific signs deserve extra attention. Catching these patterns early prevents the problem from spreading to other files or devices on the same network.

1. Pop-ups and ads appearing out of nowhere

If ads show up even with your browser closed, or on sites that normally don't have ads, it's a strong sign of adware installed without your knowledge — usually bundled with another free program.

2. Programs opening or closing on their own

Erratic behavior — windows opening without interaction, the cursor moving on its own, programs repeatedly crashing — signals something is running in the background outside your control.

3. Fan spinning up hard with no heavy usage

Cryptomining malware, for example, constantly uses processing power in the background, even when the PC looks "idle." If the fan kicks in hard with few programs open, it's worth investigating.

4. Browser homepage changed on its own

Browser hijacking typically changes your homepage and default search engine to redirect traffic to third-party sites, usually installed alongside an unwanted extension.

5. Contacts receiving strange messages from you

If contacts get suspicious links from your email or social accounts that you didn't send, it's a sign of a compromised account — change the password immediately and enable two-factor authentication.

6. Disk space disappearing without explanation

Some malware types create copies of themselves repeatedly. If free space keeps shrinking without you downloading anything relevant, investigate what's consuming it.

7. Antivirus turning itself off

More sophisticated malware tries to disable Windows Defender to keep operating undetected. If your antivirus shows as disabled and you didn't do it, that's a serious warning sign.