I wanted to call your attention to a critical, out-of-band Microsoft Security Bulletin released today. Here's an excerpt from it (the emphasis is mine):

This security update resolves a privately reported vulnerability in the Server service. The vulnerability could allow remote code execution if an affected system received a specially crafted RPC request. On Microsoft Windows 2000, Windows XP, and Windows Server 2003 systems, an attacker could exploit this vulnerability without authentication to run arbitrary code. It is possible that this vulnerability could be used in the crafting of a wormable exploit. Firewall best practices and standard default firewall configurations can help protect network resources from attacks that originate outside the enterprise perimeter. This security update is rated Critical for all supported editions of Microsoft Windows 2000, Windows XP, Windows Server 2003, and rated Important for all supported editions of Windows Vista and Windows Server 2008. For more information, see the subsection, Affected and Non-Affected Software, in this section.

Because the vulnerability is potentially wormable on those older versions of Windows, we’re encouraging customers to test and deploy the update as soon as possible.

Please check the links below for additional details:
http://www.microsoft.com/technet/security/Bulletin/ms08-067.mspx
http://blogs.technet.com/msrc/archive/2008/10/23/ms08-067-released.aspx