Avast! IMO, a better, simpler interface to work with. I have heard great things about AVG as well, but I will not change from Avast! until I get infected, and it can't remove it. I always recommend Avast! for a free antivirus solution.
the last time i used avg was about 4 years ago and it let my dads computer get infected with a bunch of email viruses but that was back when avg 5 was the latest i think, so maybe i'll try avast and see how it does.
and my dads friends keep telling him that avg is one of the worst antiviruses out there and the guy that said that works with alot of computer stuff