Courtesy of Donna's Security Flash.
After a period of successful beta testing, Microsoft Security Essentials (MSE) is final! MSE is independently certified by West Coast Labs and is backed by Microsoft's global security response team. Most importantly, MSE is built on the same core security technology used in Microsoft Forefront, the enterprise version for businesses.

MSE is free for personal use and can be downloaded directly from the dedicated Microsoft web site at http://www.microsoft.com/security essentials.

In the event you are replacing a security suite with Microsoft Security Essentials, it will be necessary to install a replacement software firewall or activate the Windows Vista or Windows 7 Firewall. Online Armor Free and Agnitum Outpost Firewall are free for personal use.
More good stuff here -
Microsoft Security Essentials ~ Security Garden