I planned to mention grisoft's AVG antivirus program as well. The program updates automatically when necessary and even these updates are free. So I'll second your recommendation.
AVG is the best AV software I've found. I ran it, Norton, and McAfee side by side for a month and AVG consistently found and cured virii that the other two either missed altogether, or found but couldn't fix. And AVG is free for personal use. Can't beat it.
MM