We're currently tracking the Sasser worm and we're posting updated information on Microsoft.com as we get it. Check out the following page for information related to this worm:
What You Should Know About the Sasser Worm
Microsoft teams and law enforcement authorities are investigating reports of a worm, identified as W32.Sasser.worm, that is currently circulating on the Internet. Microsoft has verified that the worm exploits the Local Security Authority Subsystem Service (LSASS) issue fixed in Microsoft Security Update MS04-011 on April 13, 2004.
We're linking to the TechNet page on the MSDN Developer Centers