Enabling hardware video acceleration = c ...

I'm not sure what it is, but it seems that everytime that I try to enable hardware video acceleration in any given program (such as Nero Showtime or WMP), the following things happen:

• In every DVD player program I've tried, if I enable the hardware video option, I suddenly can't see the video. e.g. when I enable it in Nero Showtime and play a DVD movie, it gives a gray screen while I can hear the audio.
• When playing back a video file that came from a TV show, the color channels seem to go way off when playing a file with hardware video enabled in WMP.

So why is this?