I havent always been a big fan of anti virus software coz of the way they take over my pc's and rediculous uninstall nightmares associated so i've used online scans usually from symantec's webby then uninstall the nasties manually but i found a really neat av package called NOD32.
it has a very small footprint on my system and has found a few dubious files that the online ones missed.
strangely enough it came with a 1 million day trial but i think i'll subscribe once this has run out