Why do I get an error when trying to Play a DVD?

Could be a lot of reasons.

Make sure no other application is opened that uses an Overlay surface (Any program that is capable of playing Media Files, such as Kazaa), etc...

DVD playback can take a lot of CPU processing power, so it's wise not to run any other application if your CPU isn't very strong.

Make sure you use the Smart DVD Playback mode introduced in version 3.0 to prevent filter conflicts.

It's also possible that at a certain resolution your display card doesn't support the Overlay Interface. The Overlay Interface is mandatory for DVD playback (unless a VMR mode is used), but optional under Media playback. So while media playback may work, DVD playback will not. The solution is to lower your display resolution. For example, on a TNT2 card with 32mb ram, the maximum DVD resolution is 1024x768@16bit. At 1024x768@32bit, the card is unable to play DVD content anymore. This varies from card to card with some of the newer cards being able to work at 1600x1200 with no real issues.