I use Avast!, is similarly free to AVG.
My (well out of date) experience of AVG was that it missed two that Norton did pick up, while much more recently, Avast! picked up a trojan that had been sitting in a 6 year old file!

As mentioned by others, one of the best things you can do is use Firefox with noscript + adblock plugins.
I have java disabled + cookies disabled with manually set exceptions for sites that I want to let give cookies ie forums & webmail etc.

Also have, keep up to date with fairly frequent scans: Adaware, Spybot S&D + Spyware Blaster.

For certain types of infection, smitrem is about the only solution short of full format.