We [Microsoft] tend to have quite a few diamonds in the rough. One of those is our DLL Help database. This is a public tool where you can search to see nearly all of the public releases of a specific file. For example, if you are having an issue with a Microsoft product and have it narrowed down to a DLL, EXE, or SYS file, then you can look it up to see if there are newer releases of it and/or what public release the file was installed by. Anyway, here is the location of the database.

Microsoft DLL Help Database
http://support.microsoft.com/dllhelp/

The disadvantage of this database is that it doesn't always show *all* of the public releases such as hotfixes. For that you need to search the Microsoft Knowledge Base for the file name located at:
http://support.microsoft.com