At 10 a.m. Pacific Time today, Microsoft will release an emergency security update for Windows. The details of the vulnerability were not revealed in the Advance Notification Bulletin that Microsoft released late last night, but we can assume it's a significant one for Microsoft to go "out of band" and release it before the next scheduled Patch Tuesday, two and a half weeks from now. Out-of-band updates have been rare since Microsoft instituted the regular Patch Tuesday schedule.

The Advance Notification states that the vulnerability affects Windows 2000, Windows XP and Windows Server 2003 and is "critical" for them. It also affects Windows Vista and Windows Server 2008, but is rated "important" for those operating systems.