You need to click save to my computer on movie maker. This
will convert it to WMV make it viewable on the computer and
compatible with youtube

When saving on WMM, to have it as the right file for
uploading onto youtube you need to do the following: File,
Save Movie File, Hight light "My computer then click Next.
Enter the file name you wish it the be and the place you
wish to save it in then next and next again it will start to
save your file under the right file(WMV) for uploading
then.

All you will need to do after the svaing
is complete, go back to Youtube uploading and do the same as
before with the new file.

I think there's an option with Microsoft Movie Maker to
export the video to the WMV format. But others would
probably be able to tell you more precisely.