Just followed the steps here to fix up my windows media player library, as it wasn't letting me create auto play lists today.
Have included the steps below in case the above web site stops running...
Sometimes a Windows Media Player database can become corrupt for any number of reasons. This will obviously also effect Media Center since it runs off the same database for videos and music. Luckily fixing the problem isn’t too difficult.
First thing that needs to be done is to stop the Windows Media Player Network Sharing Service. This service controls media sharing over your network but it also locks the database file so it can’t be deleted by the user. To stop this service enter task manager (crtl+alt+del) and select the services tab. Find “WMPNetworkSvc” in the list then right-click and select “Stop Service”. If you receive a denied error you can also stop the service from the “Services” button at the bottom.
Once the service has been stopped open Windows Explorer and go to “\Users\YOUR USERNAME\AppData\Local\Microsoft\Media Player”. It may be hidden so you won’t be able to see it but you can type it into the address bar to access the location. Alternatively, you can also enable hidden folders to be shown on your pc by going to Control Panel > Folder Options and enabling showing hidden folders.
Once you’re in the folder, locate the “CurrentDatabase_371.wmdb” file and delete it. If you receive a permission denied error the “WMPNetworksvc” may not have been stopped or you may need to restart your machine and start again.
Once the wmdb file has been deleted, restart Windows Media Player and the database should begin to repopulate. It may take some time to complete depending on the number of files. That should be all there is to fixing your corrupt Windows Media Player database.