I have been trying the MS Security Essentials for some time now. I am not sure how astute it is in finding computer viruses (as apposed to the competition, i.e. Norton, McAfee, Sophos), but so far, so good. I am very conservative in my searches and general web browsing, so maybe I am not the best tester. However, the interface for the free Microsoft anti virus software is very nice and simple to use, so if you are on a tight budget, it may be worth a look.
Microsoft Security Essentials
"The anti-annoying, anti-expensive, anti-virus program"