Windows Media Plyaer and DVD playing

I have WinXP Home and the Windows Media Player that came with it. When I asked I was told that I should be able to play a DVD with WMP. But when I try to play it says I need a decoder or it says cannot play try to decrease the number of colors or the screen res. Why is it doing this? If I use my other DVD player, InterVideo WinDVD4 it plays it, with issues of course.

 

Can you copy/paste the whole error message for me,the more info you can provide,the easier it is to help you.