When I tried to make a test movie from WMM, saving using the Web option, it is saved as a .WMV file. There are many free Video Converters available
here for you to use to convert your files. I suggest filtering the results by License Type (choose "Free") so that you don't end up with trialware that only works for a couple weeks. At the top, choose either Sort by: Editor Rating or Sort by: Total Downloads in order to sort by the most popular or best software. Although I haven't personally used it,
Any Video Converter seems like it would be a good pick. For file types YouTube accepts, go
here.
Before trying any of the above:
YouTube Help wrote:
Windows Movie Maker, by default saves projects as .MSWMM files. These file are project files, which means it only tells Windows Movie Maker the layout of your video and does not contain the final video itself, and because of this, YouTube does not accept .MSWMM files. To upload your Windows Movie Maker video to YouTube, select the "Save to My Computer" option to save the file and upload the generated file.