No details as to what it involves, but here are a few possibilities.

Microsoft on Thursday announced plans to ship one security bulletin on Tuesday, Sept. 13, to provide patches for a "critical" flaw in its Windows operating system.

As part of its advance notice mechanism, the Redmond, Wash.-based software giant said the security update will require a restart and can be detected with the MBSA (Microsoft Baseline Security Analyzer) tool.
http://www.eweek.com/article2/0,1895,1856939,00.asp
EWeek :: September 8, 2005