How does antivirus software work?

Scanner (conventional scanner, command-line scanner, on-demandscanner) - a program that looks for known viruses by checking forrecognisable patterns ('scan strings', 'search strings','signatures' [a term best avoided for its ambiguity]).
* TSR scanner - a TSR (memory-resident program) that checks forviruses while other programs are running. It may have some ofthe characteristics of a monitor and/or behaviour blocker.
* VxD scanner - a scanner that works under Windows or perhaps underWin 95, or both), which checks for viruses continuously whileyou work.
* Heuristic scanners - scanners that inspect executable files forcode using operations that might denote an unknown virus.
* Monitor/Behaviour Blocker - a TSR that monitors programs whilethey are running for behaviour which might denote a virus.
* Change Detectors/Checksummers/Integrity Checkers - programs thatkeep a database of the characteristics of all executable files ona system and check for changes which might signify an attack byan unknown virus
Cryptographic Checksummers use an encryption algorithm to lessenthe risk of being fooled by a virus which targets that particularchecksummer.