First it is Movie Maker 2.1 not 5.1 (5.1 is the XP version #)
As far as your issue the video is probably an MPEG or MPEG2 video which
doesn't work well if at all with Movie Maker.
Convert it to AVI or WMV and you should be ok again.
For conversion software and info visit this website: http://www.videohelp.com/convert