I've used Avast! a lot in the past simply because there was nothing else that would run on Vista during beta testing. It seems fine and isn't a resource hog. Some feel using it is just kidding yourself that you're protected but I don't agree with that.
Another free anti-virus I like is Avira AntiVir - it has a record almost as good as that of NOD32. The only issue I have with it is that each time you startup the system there's a prompt to go the Avira website so I stopped using it.
"The great majority of mankind are satisfied with appearances, as though they were realities, and are often more influenced by the things that seem than by those that are." - Niccolo Machiavelli