ZD Net posted an interview with Bill Gates, one of the things they talked about is the speed of Windows updates:
Q: Does it make sense to have Windows be something that is updated like Office on a pretty regular basis, regardless of the level of innovation that's there? Should there be a new version of Windows every 18 months?
Gates: (Features) like the browser user interface, the media capability, some of those things you can have updates more often than even every 18 months and users who want that can download those things because they don't affect compatibility. Whereas the file system or the scheduler, the rights protection pieces, the device driver interfaces--those you're never going to modify more often than every three years, or in many of those cases you want to leave those things alone for way longer than that.