I'm also running Spyware Nuker XT right now, but it only has a 30 day trial. After that, they make you buy a license if you're going to continue to use it. But it has picked up a trojan on my pc that was causing a lot of trouble, and no other av's that I tried could find it, it was hidden so well.
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.
I used to like AVG except the last update I did slowed everything down until I uninstalled it. I also use NOD32, except it conflicts with Poser 7????? strange. Also A-squared is good. Yes I also run Norton AntiVirus. I find NOD32 catches the most nasty stuff. Malwarebytes is also good for catching other nasties