"If you receive this error message, you can try to lower the color depth, resolution, and refresh rate of your display driver. This error message can also occur if Microsoft NetMeeting is started or if the NetMeeting icon is on your taskbar. NetMeeting and DVD playback both use the overlay mixer. Only one program at a time can gain access to the overlay mixer. Another possible solution is that the computer is running Windows 2000 Server and Terminal Services is installed. When Terminal Services is installed, you cannot play a DVD because of high bandwidth requirements of video playback over a Terminal Services client connection."
Fra
http://www.ravisent.com/support/faq.html#8