The top 3 for windows seem to be:
Antivir - I had problems with high CPU usage when I tried it
AVG Free - Seems to work OK, but I never get viruses (accidentally)
Avast - Haven't tried it, yet
If you're not using it already, I recommend
AdAware for spyware, which is far more common if you're using IE. I've seen computers where spyware has completely taken over to the point of being unusable, and it's a rarity when I use someone else's computer without it having something nefarious on it.