It's going to be a codec issue, but download this application it basically can play anything
http://www.videolan.org/vlc/
I had the same problem in the default media player that comes on XP.
I tried Apple Quicktime (iTunes put it on this machine even though I wanted neither application) but that's just crashed as per usual leaving me a blob in the middle of my screen I'll need to reboot to get rid of
(or work out what process to kill)
That download seems to have worked.
But, a question for the jury, why do people insist on creating videos that require the end user to download additional software/codecs to watch? What's wrong with just using the generic ones that the majority of people have installed as default?