windows movie maker with vista

whenever i finish editing a movie with movie maker and i publish it to either
a cd or for playback on my computer everything goes as planned. when i try to
play it back with windows media player it will play the first 38 seconds or
so and then the video will crap out and stop showing. on the bottom it shows
that it's still playing, and in some cases the audio will continue to play,
but no video. why is this and how do i fix it so that all my work doesnt go
to waste?