I was running Norton antivirus it slowed my PC down considerably, but I put up with that to be safe, till I got 2 Trojan’s and it didn’t catch them so I gave it the flick.
Now I run Ad-Aware 6, zone alarm pro and this http://housecall.antivirus.com/housecall/start_corp.asp
PS: the house call was the only one to find and get rid of the Trojan’s and it’s free