I think this is most likely a PC problem.
The way I understand it is that Windows does not natively support mpeg2 so third party players, eg installed software DVD players, provide this.
Some work better than others and only some will playback mpeg2 clips in PowerPoint.
In other words, whether mpeg2 clips play on a particular machine is determined by what software has been loaded on that particular machine. PowerDVD is such a programme that is often preloaded on windows machines.
Don’t know about Vista mind you.
Alan