The media library is outdated or the source is no longer available, you would want to delete all the files in the C:\Users\username\AppData\Local\Microsoft\Media Player. However, all but a couple of them remain, one being the likely rather large database. This was the situation on a Windows Vista workstation with SP2.

To resolve this, go to your Services through Control Panel > Administrative Tools > Services. Stop the service called Windows Media Player Network Sharing Service. Go back to the folder C:\Users\username\AppData\Local\Microsoft\Media Player and delete the remaining files. When complete, start the Windows Media Player Network Sharing Service.