I love the free version of Avast. It was recommended to me by a friend, and since then, all the family's PCs have it installed for about 3 years now. Never had any problems with it, doesn't use up much RAM or CPU time, starts up quickly when I log into Windows XP (It also runs very quickly on Vista by the way - one of the PCs in my family has it running with Vista Ultimate.)
It has gotten rid of some pretty nasty viruses, and only once in 3 years has it not been able to properly get rid of a virus. I highly recommend it.