Most slow or odd-behaving computers aren't infected. They're old, nearly full, or overdue for updates. But a handful of signs genuinely do point to malware, and it's worth knowing which is which, because the first few minutes matter.
Signs worth taking seriously
1. Pop-ups and changes you didn't make
A new home page, a search engine you've never heard of, toolbars you didn't install, or pop-ups appearing even when the browser is closed. These are the classic signs of adware or a browser hijacker.
2. Programs you didn't install
Unfamiliar apps showing up, especially "cleaners", "optimizers" or "PDF tools". Often they arrived bundled with something you downloaded, and they're rarely helpful.
3. Your antivirus is off and won't turn back on
Real malware often tries to switch off security software first. If Windows Security or your antivirus keeps getting disabled by itself, treat that as a red flag.
4. Things sent from your accounts that you didn't send
Friends telling you they received strange messages from you, or password-reset emails you never asked for. This usually means a password has leaked, which is a separate problem from a virus but just as urgent.
5. The fans roar and the PC crawls while you're doing nothing
Some malware (cryptominers, for example) quietly uses your processor all day. This one is also caused by dust, a stuck update or too many startup programs, so don't panic yet. My slow PC guide covers the harmless causes first.
6. Files you can't open, or a message demanding payment
If your files have strange new extensions, or a window says they are locked until you pay, that's ransomware. Switch the computer off or disconnect it from the internet straight away, and do not pay.
Signs that are usually not a virus
- The computer is slow right after a Windows update. It often settles down within a day.
- A single website shows a scary "You have 3 viruses!" pop-up. That's the website, not your computer. Close the tab (or the whole browser) and don't click anything inside it.
- The laptop gets warm while it's charging or playing video.
- One program crashes. Other programs are fine.
What to do in the first ten minutes
- Disconnect from the internet (turn off Wi-Fi or unplug the cable) if you entered a password on a suspicious page, installed something you now regret, or let someone connect to your computer remotely.
- Stop doing anything sensitive on it. No online banking, no shopping, no logging into email until it's checked.
- Run a full scan. In Windows Security, go to Virus & threat protection > Scan options > Full scan. Then get a second opinion with Malwarebytes Free, which catches things other tools sometimes miss.
- Change your passwords from a different device, such as your phone. Start with your email, then your bank, and turn on two-factor authentication wherever it's offered.
- Call your bank immediately if you entered card or MB WAY details anywhere you now doubt.
When to stop and ask for help
If there's a ransom message, if someone had remote access to your computer, if scans keep finding the same thing, or if you simply can't get rid of the pop-ups, it's time to get a second pair of eyes before things get worse. I can look at it remotely or in person, and I'll tell you honestly what it needs.
Get in touch and I'll take a look →
Related: how to spot a phishing email and MB WAY and SMS scams.