How to Tell if Your PC Has Malware: 10 Warning Signs

By Thomas Løvaslokøy · Published May 31, 2026 · 8 min read

A computer rarely announces an infection. Modern malware is designed to stay quiet — stealing data or hijacking resources in the background — so the warning signs are often subtle. This guide lists the ten symptoms that should make you suspicious, shows you how to confirm an infection rather than just worry about one, and tells you exactly what to do next. If several of these signs ring true, the fastest way to know for sure is a full scan with a reputable antivirus such as Bitdefender.

10 warning signs of malware

No single symptom proves an infection, but the more of these you notice together, the more likely something is wrong.

• Sudden, unexplained slowness. Your machine bogs down even with few programs open, because malware is consuming CPU, memory or disk in the background.
• Pop-ups and unwanted ads, especially ones that appear outside the browser or on sites that normally have none — a classic sign of adware.
• Your browser changed by itself. A new homepage, an unfamiliar default search engine, or toolbars and extensions you did not install.
• Programs you do not recognise, launching at startup or running in Task Manager with odd names.
• Your security software is disabled or will not turn back on — malware often tries to blind your defences first.
• Frequent crashes, freezes or the blue screen, beyond what the machine normally does.
• Unexpected network activity, with the disk or network light busy while you are doing nothing — possibly data being sent out.
• Files encrypted or renamed, with a ransom note — the unmistakable sign of ransomware.
• Friends receive messages you did not send, suggesting an account or the machine has been compromised.
• Redirected searches, where clicking a result sends you somewhere unexpected.

How to confirm an infection

Suspicion is not proof. To confirm, run a full scan — not a quick one — with a reputable, fully updated antivirus. A quick scan only checks the usual hiding spots; a full scan examines the whole system. If your installed antivirus has been disabled or you do not trust it, run the scan from Safe Mode, where most malicious code is prevented from loading, which makes detection and removal far more reliable. It can also help to run a second, on-demand scanner as a sanity check, since no single engine catches everything. Check your list of installed programs and browser extensions for anything you did not add, and review startup items in Task Manager. If a scan flags threats, you have your answer; if it comes back clean but the symptoms persist, the behavioural signs above still warrant the removal steps below.

What to do if you find malware

If a scan confirms an infection, act methodically. Disconnect from the internet to stop data being exfiltrated or further payloads downloading. Let your antivirus quarantine and remove what it found, then reboot and scan again to be sure. Change your important passwords — email, banking, anything sensitive — but do it from a different, clean device, since an infostealer on the infected machine could capture the new passwords too. A password manager makes that mass reset far easier. For the full, careful process, follow our step-by-step malware removal guide. If the infection turns out to be ransomware, see our ransomware protection guide before doing anything else.

How to prevent it next time

Most infections are preventable. Keep your operating system and software updated so known vulnerabilities are patched. Run a reputable antivirus with real-time and web protection turned on — our best antivirus roundup covers the options. Be sceptical of email attachments and links, avoid pirated or cracked software (a leading malware source), and only install apps from trusted sources. Use strong, unique passwords with two-factor authentication, and keep regular backups so that even a worst-case infection cannot destroy your data. Those habits, plus one good security product, block the overwhelming majority of threats.

Common mistakes to avoid

• Ignoring the early signs and hoping they pass — malware rarely fixes itself and often spreads or escalates.
• Calling the number in a pop-up. "Your PC is infected, call this number" pop-ups are themselves a scam; never call them.
• Changing passwords on the infected machine. Use a clean device, or you may hand the new passwords straight to an infostealer.
• Running two real-time antivirus products at once to "be safe" — they conflict. Use one, plus an on-demand scanner if you want a second opinion.
• Restoring from an infected backup, which simply reinfects you. Use a backup from before the symptoms started.