My Norton is about to expire, and MS just released MSE the day before yesterday, so I quickly gave it a try.
Microsoft Security Essentials (MSE) can protect computers running Windows XP, Vista, and Windows 7 against viruses, spyware, rootkits, and Trojans. The Microsoft Security Essentials software itself cannot fend off all web threats, but when used with the IE8 browser and a fully updated Windows operating system, it will provide users with a complete security solution.
This product is free to use and will replace the previous paid antivirus product OneCare, aiming to encourage users who have not yet used security software to use security software as much as possible.
The official MSE page: http://www.microsoft.com/security_essentials/
It seems the download link is no longer visible now. So I found a few direct download portals: XP 32-bit VISTA / 7 32-bit VISTA / 7 64-bit
[11.21 update] Official download page http://www.microsoft.com/security_essentials/Default_zh_cn.aspx