Running Windows Vista? Then you're going to want these two updates, neither of which is on Windows Update at the moment:

December 2007 Windows Vista Application Compatibility Update
The December 2007 Windows Vista Application Compatibility Update is a package of software updates that address common application compatibility issues in Windows Vista. This update is cumulative, and it supersedes update 935280. In addition to the fixes that are contained in update 935280, this update improves support in Windows Vista. This update also includes fixes that are contained in update 932246.

An update that improves the performance, responsiveness, and reliability of Windows Vista is available
This update improves performance, responsiveness, and reliability of Windows Vista in various scenarios.

Note that neither of these updates will install if you're running the SP1 release candidate.

Thanks Raf!