I'm not using any anti-virus software myself. On my MacBook Pro, it's not necessary, and I'm also not running any on my Windows Vista machine. I mainly use my Windows Vista machine for photo editing and some light web browsing. As long as you don't visit dodgy websites and don't install strange software, I don't think the risk of getting a virus is very high. I do have a firewall setup properly. I don't like anti-virus software because it takes up resources and makes the computer slower.
On the computers of some of my friends, I've installed
avast! anti-virus. It's free and works well enough, I've never seen a virus on their computers.