What would be cool is a anti-virus program that nevers need updating. It'll be like a human, like an immune system. It would grow as the older it gets. So if a virus comes around it'll know, because this isn't what that person does on the PC.
Nice thought, but we will always need to update our Computer's immune system just as we do with our human immune system. Vaccinations and antibiotics are just a few of the things we humans use to "update" our immune system. Virus scanning software does some adaption with heuristics, but there will always be that new, never seen before bug out there that will need some intervention, whether it be a new drug or a new dat file.

Heuristics attempt to detect unknown viruses and often employ generalized signature scanning geared to detect "families" of viruses. If the virus is related to a known family, heuristics will detect it and report it as suspicious or infected with an unknown virus.