I use "Nightly Backups" as my anti virus. a couple months ago I tried one of them... I think it was Semantic or some such... and it was actually worse than the worst virus I've ever had... oh man.... I had to revert to a previous night backup to get rid of it.... so now every so often, when I start noticing some rather accurate add-banners, or weard hdd activity
... I just go to one of these online Trojan/worm scanning websites, scan me memory (not the drives, oh no... never do that).
and if all is well I just give spybot a spin or two... fixes it up about 99% of the time...