Avast sucked for me. It may have been because someone else had set it up before I got the computer, but it gave me nothing but grief (and then it was an absolute mission to take it off when I finally got sick of it).

I've used AVG for most of the the time that I've owned a computer, so I can't really recommend anything else for you. I think Norton is supposed to be ok, except I've only ever used trials that came on computers I bought.