mpeg files show shorter time on XP

No MPEG files will play correctly on my Dell optiplex gx270 with Windows XP. They play, and everything looks fine, but the timeline only shows between :15-:45 on what should be 30 minute movies. I can scroll through the timeline, and the whole file is there (e.g. if I go to the middle of the timeline, it is the middle of the movie), but once it reaches the end of the timeline (in realtime) it stops at whatever point the movie was at.
It is all my MPEG files, not just 1 or 2.
I tried the identical files on another computer not running XP and they work fine, although that doesn't prove it is XP causing the problem, it makes me think that's possible.
Other facts to help diagnose:

- AVI, WMV, etc. play perfectly and show time properly.
- I tried using several other players (BSPlayer, Quicktime, RealOne), all do the same thing.
- I have WMP9 and downloaded the most updated codecs.

I usually can figure these things out searching BBS or google, but nobody seems to have any info on this problem
Any help would be greatly appreciated - I'm working on a home video for my parents and it would be great to be able to get this working!

 

What type of mpegs are they?
What are you using to view them?
Do you have an mpeg2 decoder codec installed?
The reason I ask, the WMP mpeg2 codec is your problem. You need to replace it with a different one, usually provided with DVD playing software such as PowerDVD and WinDVD.