I have Avast! on all of my computers bar the netbook, which has Avira, as Avast! wasn't happy working on it. I've never had a single Virus, Avast! I don't know it is there until I get told that it has been updated or it has detected a virus, so disconnected me from the webpage/download which has it.
Avira however, occasionally it will pin my CPU at 100%, not often, but from time to time it will do, which it really should not be doing