Can antivirus remove a virus that is already on my computer?
Usually yes: a full scan with a good engine removes most common infections (adware, trojans, miners). For stubborn cases, run a second-opinion scanner like Malwarebytes Free, then a boot-time or offline scan like Microsoft Defender Offline. Ransomware is the exception: encrypted files cannot be rescued by removal, only by backups.
Antivirus is better at prevention than cure, but cure usually works. Step one on an infected machine: 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 infostealers: if one ran on your machine, assume passwords are compromised and change the important ones from a clean device, starting with email and banking.