ESET or Norton antivirus if you want to pay.
But if you're paying, I strongly suggest you buy ESET antivirus, but after ESET pack, the best is AVG which is free and available to download.
Well if you don't mind a resource hog, go with Norton Internet Security 2008.
It has one of the best scanners, I use it, never been infected. Now your choice is whether you want security over performance or performance over security
It is, but the free version of Avira uses the same Heuristic detection pattern as the paid version, just is alittle slower scanning, giving little reason to buy it.