problem playing DVDs

I use Win XP Pro with SP2 installed(no known problems) and a Sony DVD RW DW-U18A. My video card is a Radeon 9600XT.
Whenever i try to play a DVD i get a message saying that my resolution needs to be reduced. (i run 1280 x 960, 32bpp). When i adjust the resolution down to 1024x768 32bpp the dvd will play fine. i can then adjust the resolution back to my original settings.
what causes this and is there any way to make WMP play a dvd without having to adjust my resolution at least twice??
thanks