Try using the latest version of windows media player to test mpg files. It won't play dat or vob. If it plays the mpg's fine (ie use the prog to browse for the file, don't doubleclick the file to start the player), then you know your video and audio hardware are probably ok. If it doesn't play properly, then you most likely have a hardware issue with your motherboard or sound card of some sort.
 
Personally I would not have both Powerdvd and Xingdvd installed at the same time, as they may be conflicting with each other. My personal feeling is that Powerdvd (ver 2.5 or higher) is the better prog. I haven't had a single problem with it so far.
 
These suggestions may help, but I have the feeling they may not. I have seen too many posts from people with !Live issues to eliminate that card as a possible for your problems. If you know someone who has a vortex2 based card -say Turtle Beach Quadzilla or Montego II- they are both cheap and have worked great in Win2k with the Aureal 8830 drivers, you might find your problems have gone away. Then again you might not want to reinstall the !Live drivers, as that is a big pain also.

i'd also have to go with PowerDVD 2.5+, no problems whatsoever.