decoder error

I need a little help with my media center. I havent used the center for a while to watch any movies. Today I put a movie in the MC,and after clicking on play dvd, the little Box in the left corner comes up, When I double click on ,it informs me with these wordings.

(Decoder Error)
The video decoder has eather Malfunctund or is not installed.
Please restart MC and/or the computer.
I have restarted the MC and computer a half dozen times,and still nothing. Can some one explane to me about the video decoder, How did it malfunctnd and how do I reinstall it. Where do I get it from. When I do get it How is it reinstalled? I hope these don,t sound like a lot of dumb questions to you ,but I sure do need ever ones help. I don't remember deleting anything,in fact I haven't used this in a year,I have been using Power DVD,and have no trouble with it.
Thanking You In Advance
Andy
PS (I have Windows XP MC Edition 2005)