Yeah, i actually experienced that problem myself, when you use WMM (windows movie maker), and you are finished with your video, then you probably know that you click 'File' then 'Save As'. This saves your movie as a PROJECT. You must save your video as a project first.
Then, after you are done saving it as a PROJECT, you have to click 'File' then 'Save as Movie File'. This saves your video as a MOVIE. This is what YouTube understands, because it sees a project as an unfinished work in progress. Once it's done saving as a movie file, wait about 5 minutes or so while it uploads on your PC, and then go to YouTube, click 'Upload', then search 'Your Videos' for the movie that you just made. Give it a title, tags, and all the necessary information and then wait while it uploads.
I hope that this information has been helpful for you. If you have any questions, please feel free to PM me.
-SOF-Zombie