Normally you need a DVD player (like WinDVD or PowerDVD) already installed in order to use Windows Media Player or some other program for DVDs. This is because these programs do not have the CSS decryption stuff required to play the disc. The DVD consortium charges license fees for this CSS stuff, so only companies that actually sell their programs can make enough money to pay them.
Does MPC have CSS decryption stuff in it? I'm not very familiar with the program.