http://www.nytimes.com/external/idg/2008/10/23/23 ... to-ru.html
For years, Microsoft has released its patches on a predetermined day -- the second Tuesday of every month -- but it has occasionally released patches ahead of schedule when bugs have been actively exploited by computer criminals.
The last such emergency patch issued by Microsoft was in April 2007, when the company fixed a bug in the way Windows processes .ani animated cursor files. That flaw was publicly known and being exploited in attack code hosted on hundreds of Web sites.