I like Avast because it's a strong, yet lite program. It has many shields..like a P2P/Torrent shield, Web shield, Email shield, IM shield etc..
I like the fact that you can do a 'boot scan' which is a fully system scan before your OS starts.
You just keep registering for a FREE account once per year and that will keep giving you a free key. You can have this your whole life for free, as long as you register once per year. (a small registration, not much detail needed)