I'd suggest Bazooka by Kephyr. It's quite good, scans REALLY REALLY FAST! Although, cant find much, just a load of adware, some worms and a handful of trojans.
All the same, I use it on top of AVG Free, SpyBot S&D, Ad Aware, Windows Defender, MRT (XP Pro) and CWShredder.
Like the paranoid person I am