Some people have found they run into problems when they publish a movie from Movie Maker in Vista. The movie compiles properly in the timeline and can be viewed without a problem. When the movie is saved though they find that maybe half of the screen is covered by a solid green colour.
There are several reason for this.
The video file you have used may be copyrighted and this is the copy protection kicking in.
You need to update your graphics card drivers, see here for how this should be done.
You are using incompatible file types in your movie, see this page.
You may need to turn off some filters that have been added by other programmes and are interfering with movie maker. Look in the compatibility tab and in Vista untick any filters that are listed and try again.
A popular problem codec is Xvid, you can sort this by
Going to <Start Menu> <All Programs> <XVid> <Configure Decoder>.
In the <Output> options, choose <Compatibility Renderer> and click <OK>