Microsoft today has patched a severe security flaw in many versions of Windows, affecting TCP/IP. The severity of the flaw ranges only slightly between software, with Windows 2000 only being vulnerable to a denial of service. The rest of the affected versions however are potentially in danger of code execution and compromise.
The affected versions include Windows XP SP2, including the x64 edition, Server 2003, Windows Vista and Vista x64 edition. Along with another lesser security patch, Microsoft has issued two updates today that fix these issues, so anyone using automatic updates will have already gotten it pushed out to them. You can read the security notice regarding the update or, better yet, actually go and update.