You cannot make a video go backwards using Windows Movie Maker. It doesn't have the capability to do so. Instead you need to use another program. Sony Vegas can do it. I think Adobe Premiere also has that capability.
However there is a trick in WMM you can do to make a video run backwards, although it is time consuming. It worked for a friend of mine.
What you need to do is zoom in as far as you can on the video, cut the portion you want to run backwards into as small sections as possible, and then rearranging them in the opposite order, so instead of the video's starting point being from left to right, it'll be from right to left, and when run, it'll be backwards. I'm not sure how efficient this would be, but it's the only way I know of to make WMM run vids backwards.