We downloaded music videos and until some days ago they were working fine. Now we get the following error message:

Windows Media Player encountered an unknown error. This can occur when another program or operating system component encounters a problem but does not communicate the nature of the problem to the Player.

Already downloaded the Windows Media Player again and also tried to play the videos in Real Player, nothing is working.
Movies that we downloaded are working absolutely fine.
Does anybody know what the problem is?