How can I get my computer to play a DVD (as if it were a TV)?

I have a HP Pavilion laptop with XP Media Center. When I insert the DVD;
-if I choose to play the DVD using Windows Media Player, I get an error
message that says, "Windows Media Player cannot play the DVD because a
compatible DVD decoder is not installed on your computer."
-if I choose to play DVD video using Media Center, I simply get a blue box
that says "Video Error".

I really don't care what it uses to play the video, I just want it to play
it. How do I make that happen? Thanks!