In a limited (very short window) public beta release just a few months ago, Microsoft introduced it's predecessor to their paid antivirus program, OneCare.
The team has been very active updating not only the core product but also releasing new virus definitions (almost daily) to beta testers using XP, Vista, and the Windows 7 RC and RTM.
Known as Microsoft Security Essentials, the new program is not intended to be a replacement for commercially available products but rather to "provide comprehensive anti-malware protection from threats including viruses, spyware, rootkits, Trojans, and other emerging threats in a single lightweight anti-malware solution".
The program so far has been effective and uses a minimal amount of system resources and sports a clean and easy-to-use interface (pictured below):
In a letter to beta testers just a few hours ago, Microsoft asked that everyone update to the latest core product (version 1.0.1500.0) prior to the public release 'in a few weeks'.
It seems likely that the launch of Security Essentials is being timed close to, or just before the full public release of Windows 7 on October 22nd.
Additional information on Microsoft Security Essentials can be found on the original page launch page here along with links to a few open forums. (Note: The download link in non-functional at this time).
Members of Microsoft Connect can also access an information channel.