On October 22, 2008 - Microsoft released security patches for all versions of Windows. This "out-of-cycle" patch was released in Microsoft Security Bulletin MS08-067 . It's sudden release only serves to emphasize it's importance.

The bulletin warns of a remote code execution attack that can spread rapidly across the Internet. Evidence of this RPC attack was first found two weeks ago and infects machines without any user interaction.

Further details can be found in the following link, together with links for different versions of Windows OS.
http://techtalk.pcpitstop.com/2008/10/24/emergency-security-patch-issued-by-microsoft/

Don't delay - do it today :-)