I used to use AVG free till they updated it to version 8 and I found it slowed my PC down something terrible. Could be that I didn't have something set up properly - dunno - but that was when I started my hunt for another anti-virus - and I think I've tried most of the "big" ones and I haven't liked any of them - apart from Avira - which I've had on trial so not had to pay for it yet!!! I must admit, I like having the anti-virus/firewall, etc all in one - it's less hassle. Mainly cos I can't remember which firewall I liked - Comodo, PC Tools, Agnitum and whatever else I tried. One of them I had a bit of a problem with uTorrent on it and I can't remember which one.
The guy in PC World made a half hearted attempt at trying to convince me that Norton was a lot better now than it used to be. I wouldn't take him up on it though.
I'm in the process of setting up the laptop just now - geez Vista looks a lot different to XP at the start anyway. I feel like a complete newbie trying to use it!