There have always been a few free anti-virus products out there. They lure you in with a base product and then try to upsell you to a more configurable product. AVG antivirus is probably the best known.
Well, Microsoft is about to become one of the biggest AV software providers out there. Microsoft Security Essentials is now in beta. This replaces the One Care product that I don't think was overly successful in the market place. However, MS Security Essentials is free, and that's an important distiction.
Only 75,000 downloads of the beta are allowed here: http://www.microsoft.com/security_essentials/market.aspx.
I'm in Canada. So, I can't try it out. Only for the US, China, and Brazil.