It’s not clearly documented on the YouTube site but you can upload WMV files to Youtube.

Here is a quote from the Youtube website:

What video file formats can I upload?
YouTube accepts video files from most digital cameras and camcorders, and cell phones in the .AVI, .MOV, and .MPG file formats.

I found that the best setting in Windows Movie maker to create WMV files for Youtube is 320X240 at 512 Kbps.