MalwareBytes is pretty good for scans and such, but it doesn't have the features of a payed program. It also doesn't run constantly like AVG, only when you open it.
Also: check this sticky; http://sythe.org/showthread.php?t=739625
Common sense is the best antivirus. I use Symantic because i get if free but I don't even need it. I used to use Avira or something. I've tried a lot of them before just because.