I have had a couple people ask me why Windows Movie Maker has such a poor quality output, and I just tell them it, actually, has quite a good quality output. You just have to know how to make it good. I learned all this info from an external tutorial on the web and decided I should import it to this website.
When you are exporting your movie file, you will soon come to a part that says "Movie Setting" at the top. This is where you tell it what quality you want it to have. Select the option "Other settings:" and scroll down the list to find the best quality for you. I believe that the choice "High Quality Video (large)" outputs the very best quality you can get on Windows Movie Maker, but correct me if I'm wrong.