Microsoft Security Essentials. Not many people know about it buts its a great program, and doesn't bog your system down.
This has rated the top antivirus software for 2010.
http://anti-virus-software-review.toptenreviews.com/
I've used Kaspersky and AVG (AVG is free) and found them good and 'light' on resources.
i use McAfee... works great
Norton would find like 16 virus in a scan and then not fix them, would scan 2 times in a row and same ones come up.
And McAfee i could run in the middle and it would find 0-1 virus's lol.