I'd like to request that all members running windows please run Windows Update as soon as possible.
Most versions of Windows are vulnerable to a remote code execution attack using a specially crafted TCP/IP packet. Currently Denial of Service has been seen, but it could potentially lead to full compromisation of some versions of Windows. Windows Update has patches for the versions of XP, 2000 and 2003 that are vulnerable, unfortunately there is no patch yet for Windows 98.
Note that most firewalls will likely catch the malformed packet, but better to patch anyways.
More details: http://www.microsoft.com/technet/security/bulletin/MS05-019.mspx