Later today Microsoft is expected to release an out-of-band (OOB) update to patch a critical flaw in all supported Windows versions.
Full details on the exact nature of the flaw is, as yet, unavailable (publicly), except that it is a Remote Code Execution flaw and will require a restart. Premier Support customers have already been notified and public knowledge should be available in the next few hours, with the update being made available through normal update mechanisms later today.
OOB releases like this are generally due to severe and actively exploited flaws.
According to sources, this update will NOT be made available for Windows Server 2003 since that server OS version exited Microsoft supportability on July 14 this month.
Stay tuned. We'll let you know more as the day progresses.