System Update Readiness Tool (CheckSur)

"System resources, such as file data, registry data, and even in-memory data, can develop inconsistencies during the lifetime of the operating system. These inconsistencies might be caused by various hardware failures or might be caused by software issues. In some cases, these inconsistencies can affect the Windows servicing store, and they can cause software updates not to work. The System Update Readiness Tool tries to resolve these inconsistencies."

For Vista, Server 2K8, and Windows 7.

This has been helpful in the past for failing installations of .NET and related patches. Takes quite a while to run.