I'm using it now, was one of the lucky 75,000 people to get it.
Its really light on resources and it very easy to use and configure. Its designed to replace Defender, though it does not turn it off, but in future versions it will do so, according to Microsoft. It uses Windows Firewall.
http://www.microsoft.com/security_essentials/
Edited by deleted (Wed 24-Jun-09 23:03:04)