I had this same problem and I was able to trace the cause of mine. I wanted to store document and program files on my D partition. To do this I had to create a new user for myself, so the OS would automatically store those files on D. Then a couple of months later I made a slideshow for a group I"m involved with and ran into the "class not registered error." To resolve this issue I had to locate the Windows Media Player program file in the Programs folder on C drive (the filename is wmplayer), and double-click it to launch the program. It then started adding the media files (audio, video, pics, etc.) to the new account I created under D drive. After this I was able to open and play these files with WMP.
Hope that helps someone out there.