Hi Martin,
You may need to rebuild the Windows Media player library database.
Here is how:
a. Exit Windows Media Player.
b. Click Start, click Run, type %LOCALAPPDATA%\Microsoft\Media Player, and then click OK.
c. Select all the files in the folder, and then click Delete on the File menu.
Note: You do not have to delete the folders that are in this folder.
d. Restart Windows Media Player.
For further troubleshooting, refer to the article to run the troubleshooter that will reset the settings:
http://windows.microsoft.com/en-IN/windows7/Open-the-Windows-Media-Player-Settings-troubleshooter
Please let us know the results.