one of the best i have tried so far is Symantec Antivirus 10 (none of that norton crap) this is a professional AV, used by corporations, and may cost a bit to get a lisense, but i guarenty it is really worth it.
reason not to use symantec...... : my old harddrive... a 20 gigger from about two years ago... was set up as a slave at my firends house becuase we were going to format it.. but first we thought it would be funny to scan for virus's and what not.. well.. in the end.. symantec ended up finding 7... AVG found 85.... ur call...
Id go with AVG or Avasti, I love AVG, the best AV's aren't to be payed for they are free, just like spyware programs most of the good ones are betas and or are free.
i like the Zone Labs, Zone Alarm with Antivirus is very good. E.X: My dad works for gov and they give him norton antivirus corp, i said to him u have an active x virus and he said norton didnt find anything so i install zone and find two trojans beond repair. enough said.