Microsoft Urges users to block ports until a patch is ready, but workaround cripples browsers. The ports that need to be block are 139, a common NetBIOS port and 445, a common SMB over TCP port. The vulnerability hits Windows 7 and Windows Server 2008 R2 both 32 bit and 64, but not older version of Windows. The newly found bug was discovered by Laurent Gaffie and details were posted on the Full Disclosure mailing list.
Here is more coverage of the zero-day exploit:
http://www.pcworld.com/businesscenter/article/182257/microsoft_provides_guidance_on_windows_7_zeroday_vulnerability.html
http://computerworld.co.nz/news.nsf/scrt/E9592E1A9719742ACC25766F0066B38D
http://www.dailytech.com/First+Windows+7+ZeroDay+Exploit+Airs+But+Danger+Isnt+Great+For+Most/article16817.htm
How to protect your computer from the Win7 exploit http://www.networkworld.com/news/2009/111209-protect-your-pcs-from-windows.html