All the viruses do not do that, though some do, like Alcan/Alacra/P2p Worm, SDBot, some rootkit infections, Boran, Brontok etc. Generally AVs take care of the main file [ie the installer], but I have never seen that a AV can clean the infected machine completely and the infection remains in a strength enough to wreck your PC. AVs are generally ineffective against Driver rootkits like Microsoft API Helper [its a malware and not from Mocrosoft]. You need specialized tools and people to clean your PC.
To cut the long story short, there is no commercial AV available which even touch the drivers and hence can not or does not tackle driver based malwares.
One more example is Rustock, this crappy is out there for quite a long time, but none odf the AV can either recognize or delete it.
This is the reason why it is said that Prevention is Better than Cure. You need all sorts of security application running beside your AV like a Firewall, AntiSpyware [Spybot S&D with Teatimer enabled], SpywareGuard and SpywareBlaster. MVPS Hosts Files, IE Spayed and McAfee Site Advisor.
Had the AVs been enough you would not need all these.