First off, which version of Media Player and Power DVD are you using?? Most of the time, reinstalling either program will fix the problem. If you don't have Media Player 7, go to Windows Update and download it. If this is the case, and you can't find where I'm talking about, just post back, and I'll edit this message to tell you how to do it.
If this doesn't work, just post back, or PM me, and I'll be glad to assist. Good luck.