Re-install the software that came with your DVDROM. It
should've come with PowerDVD or a similar software DVD
player. DVD's use MPEG2 which is licenced, so you need to
have software installed that is licenced to play DVDs.
If your drive did not come with any software besides
drivers and such, you'll need to purchase some. I
recommend Cyberlink PowerDVD. I think there might be a
free trial demo you can try first.