After looking in the troubleshooter, Windows Media Player does not come with a component called a "decoder". For the benefit of those who may not know what it is, a DVD scrambles the data on the disc when it is being recorded. The reasons are various, but Windows Media Player (WMP) does not come with software to decode it again. To do that, Microsoft would have to pay for the software rights to add the decoder, which it deems unnecessary as not everyone with a DVD player plays movie discs (meaning it's not worth supplying it on every WMP) and most people get a CD with the drive which has a player on it.
If you have a disc with a DVD player on it (such as CyberLinks' PowerDVD), you should install that and run it to set up the decoder. You can always switch back to WMP after if you wish.
If you already have this installed, then maybe it needs removing and reinstalling after WMP is set up on the system.