it may be a codec problem : try downloading bsplayer and nimo's codec pack (
www.google.com). Have you tried to see if its the video that's messed up? if the video plays perfectly on other computers then try installing all the newest codecs (divx, xvid, mpeg4, etc. which by the way is almost all in the nimo codec pack) or just google them. You should be smart enough to install codecs. If the video on another computer doesn't play, then it may be the video that's messed up; but that wouldn't make sense if it worked before.
just to continue with my redundent post,
install all new codecs
and if you recorded it off a tv input card or windows movie maker make sure you install those programs + right version numbers
i hope that helps