MS will be releasing a Windows patch later today. It is said to fix a remote code execution vulnerability labeled as important for Vista and critical for older versions of Windows, including XP. http://www.microsoft.com/technet/security/bulletin/ms08-oct.mspx I guess many crackers will try to reverse engineer the patch the moment it is released and that evil code may soon be used. So we'd probably all better apply the patch asap...
Everyone might also want to install the free "Belarc Advisor" which is better than using WindowsUpdate to check for missing security patches in my experience - it will find official patches for other Microsoft products you have installed.
Already available for download, select your environment here: http://www.microsoft.com/technet/security/Bulletin/ms08-067.mspx