Just wanted to put my two cents in. I installed AVG Free on the recommendations of members of this board, as well as the advice of knowledgeable tech people out here in RL. (I had been looking to replace Norton, and at the time AOL was offering a free McAfee AV) I've been using it for a couple of months now with absolutely no problems whatsoever. It's a clean interface, it does what it says it will and it doesn't seem to be a drag on system resources.
AVG also offers an Anti-Rootkit program, although I haven't grabbed it yet as I'm on the fence about the necessity.