Microsoft Security Essentials is a free antivirus software created by Microsoft that provides protection against viruses, spyware, rootkits, and trojans for Windows XP, Vista, and 7.
Downloads:
Windows XP (32 bit)
Windows Vista & Windows 7 (32 bit)
Windows Vista & Windows 7 (64 bit)
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if you need to know more, there's a pretty in depth review over at Paul Thurrott's SuperSite for Windows
i've been beta testing it since june with no problems and i've switched to it full time from avg.
edit: page for it is now live: http://www.microsoft.com/security_essentials/