do you have a copy of cyber Dvd or the k-lite codec pack that comes with a Dvd decoder?
if it reads data and music just fine, im guessing that the dvd decoder or software is not working correctly.
if you have win dvd, cyber dvd, or any other decoder and it still doesnt work check how much memory your video card has, my 8 meg wouldnt play some dvd's for some reason, but my new fx5600 plays everything seamlessly, and also came with copies of Cyber Dvd and win Dvd.
if this doesnt help I wish you good luck and I too will look for a solution for you.