It sounds like your processor can’t keep up with the footage, if it’s a really large file. Try updating your video card first and/or running any updates that Windows recommends from the Control Panel’s Windows Update.
I don’t think there is anything wrong with your PC’s processing etc.
It’s Windows Movie Maker.
You can play a video in windows media player and that same video file may either not play in Movie Maker or you will only get the audio.
It’s retarded.
From what I understand, it’s a function of how windows saves these media files.