Microsoft Security Essentials

Microsoft is finally taking responsibility of its OS security.
Microsoft Security Essentials is now Officially Released.
Users have to pass Windows Genuine Authentication before installing.
Download MSE here.
If you have already installed the Beta, the Update to official release should be pushed down via Windows Updates.
The Microsoft Security Essential Scanning Engine is based off its Corporate Counterpart Microsoft ForeFront Security.

Personally I recommend this product as

• It is FREE.
• It is a Microsoft Product hence should integrate well with the Operating System (Other Anti-Virus are known to have false positives and has a history of identifying Windows System Files as Virus killing Windows).
• It is light weight and consumes little System Resources.

Cons

• Have to pass WGA (This shouldn't really be an issue)
• May collect System Information from Systems which MAY BE personally identifiable