Itunes won't play on a pc

I have itunes installed on a 64-bit windows 7 laptop. Used to work. Now it doesn't. When you click play on a song, it the icon changes from a triangle to the two rectangles (play to pause), but it doesn't play.  Many songs. They show as mpeg-4 protected files.  Have repaired itunes, upgraded to 11. Removed it. Re-installed it.  Other applications can play.  Itunes will play movies, but not songs.  Any suggestions?