Ian Dixon, over at The Digital Lifestyle, spotted an important new Hotfix from Microsoft for Windows 7 Media Center users. There has been a bug within Media Center that if the application is not shut down properly it can cause the music library to completely rebuild itself the next time it is launched. If you have a large library, and especially if you use the album view, the library can take many minutes to rebuild.
The fix is currently only offered as a Hotfix, so you will not be getting this via Windows Update anytime soon. If you are experiencing this issue head on over to the Microsoft Support site to grab it. Microsoft also recommends that after you install the Hotfix you should reboot your computer, and rebuild the music library. Let us know if it works for you!