You might find this link useful:
http://www.homevideo101.com/creating-videos-fo r-youtube-with-windows-maker/#comment-756
You'll probably find the first part of the tutorial is telling you stuff you already know but hopefully the last part will be of some use to you.
To be honest, I haven't tried this method myself, I usually save my Windows Movie Maker video as an .avi file and then convert it to .mpeg using a different program. I use one called CAD KAS MPEG AVI DVD Cutter 1.00 because the conversion process is really quick (Be warned though, the trial version only allows you to edit 50% of your movie). Then I upload the finished .mpeg to YouTube.