Today an extremely serious vulnerability in Windows was announced. ALL Windows XP users should apply this patch immediately. Anyone using Windows ME or Windows 98 that has installed the Universal Plug and Play service or is using Internet Connection Sharing should also install this patch immediately.

Microsoft classifies this vulnerability as "critical" and states that it's the worst vulnerability ever discovered for Windows. A remote attacker can obtain complete control over the victim's computer without having to use a password or any type of access method.

The vulnerability does not affect Windows 2000.

Detailed info: http://www.eeye.com/html/Research/Advisories/AD20011220.html
MS's Info and link to patch: http://www.microsoft.com/technet/treeview/default.asp?url=/technet/security
/bulletin/MS01-059.asp

Sean