I started another thread for the same issue and I got a good solution that worked for me so it might be worth a shot for you Windows Media Payer Wont find any Music
I have Windows 7 on my laptop and desktop. I have no problem with wmp on my laptop but my desktop suffers from the same problem as yours. There are two ways that I have found around this but they are not solutions since the problem doesn't go away.
You can either delete the music library file that wmp keeps:
1. stop WMPNetworkSvc.
2. go to C:\Users\<username>\AppData\Local\Microsoft\Media Player and delete file called CurrentDatabase_371.
3. restart service and start wmp.
Another way that works for me is to remove the music folder from the library, turn off wmp, start wmp and readd the folder.
Both methods require wmp to rescan the music library but it's the only way I've managed to get it to read my music. I also leave wmp running. The library only gets corrupted when I close wmp and start it.