wmp9 dvd playback problem

[leak-seven]

when i try to play a dvd in wmp9, it gives me a error saying it was unable to play the video and suggests me to "lower resolution, blah blah". i try to play it in real media player and it gives me another error about key exchange copy protection. Whats up?

My computer is a 1.6ghz p4, 512mb ddr ram, win xp pro os and dvd drive of course.