Can antivirus remove a virus that is already on my computer?

Antivirus is better at prevention than cure, but cure usually works. Step one on an infected machine is to update the security software and run a full scan, not a quick one, and let it quarantine everything it finds. Reboot and scan again. Most real-world infections (adware, browser hijackers, trojans, cryptominers) are gone at this point.

For what remains, escalate in layers. A second-opinion scanner sees what the first one missed because every engine has blind spots. Malwarebytes Free is the standard choice and coexists peacefully with whatever you run. Malware that hides while Windows is running gets caught by scanning before Windows fully loads. Microsoft Defender Offline (built in, under Windows Security > Scan options) or a vendor rescue USB does this. If the machine still misbehaves after all that, the honest answer is a Windows reset. Modern Windows makes a clean reinstall a one-evening job and removes doubt entirely.

Two hard limits to know. Ransomware removal kills the malware but cannot decrypt your files, which is why backups, not antivirus, are the real defense. And if an infostealer ran on your machine, assume passwords are compromised and change the important ones from a clean device, starting with email and banking.