A DVD player has to:
1) decrypt the encrypted DVD
2) decompress the video and audio tracks
If a video codec (compressor/decompressor) isn't installed on the system, the player can't play DVD's or MPEG video files.
Some DVD players include the proper codecs, some don't (such as Windows Media player)
I've been there. Everytime I reinstall my system, the DVD player is the first casualty. Luckily, my ATI installation CD has a DVD player, complete with the necessary codecs.
I remembered seeing the InterActual player on several DVD's. It might just work.
There are some freeware programs available. Download VideoLan.
http://www.videolan.org/
It's a work-in-progress, but still pretty good.