Well I had a free beta Kaspersky A/V on my main computer for past year and it was very good, sat in the back ground and never took much notice of it, never slowed the machine down, but it ran out in Jan 08 so I initially went for AVG but had a few problems and the constant updating on boot up becomes a drag.
So I tried AVAST... and on installation it checks out your whole computer before the windows start up screen and it found 2 or 3 trojans and some other spyware on my hard drive and considering I had just scanned the drive days earlier with AVG !
I am really impressed with AVAST and its FREE.