Today Microsoft is rolling out an unscheduled update. This vulnerability is critical and there are reports that it has been exploited by malware for the last few weeks. The most vulnerable systems are Windows 2000, Windows XP and Windows 2003. On these systems it is possible exploit the system without authentication. On Windows Vista and Windows Server 2008, the exploit requires authentiation to run, it would likely also lead to a Denial of Service condition due to the use of DEP and ASLR in these versions of Windows.
This is the first vulnerability that can be easily wormable in the past few years. It is very important that this update be tested and rolled out by your organization as soon as possible to prevent exploitation. The Security Bulletin can be found here.