Alain,
While I've never used Windows Movie Maker, usually the Microsoft default is WMV.
What format is the rendered file type and did you have options for RealPlayer or MPEG formats? Or better yet, are there any options for optimizing for web? I'm fishing here...
Do you know You Tube's requirements for uploading files? I would guess that a WMV or Real would be you best bet. Smaller in file size and are a streaming file - this important if you wish others to enjoy your hard work.
Progressive download files such as MOVs will work, but playback is asking a lot of a viewer, in my opinion - I loath waiting for more of the clip to download so I can continue watching a movie. Yeah, I'm jaded.
At work, we render only WMVs (optimizing for bandwidth) before placing files on a web or media server. Saving the higher rez files (AVIs and MPEG2) for DVD mastering.
It's a safe bet that other club members have performed something similar and can offer better, more practical advice for uploading to websites such as You Tube.
Good luck, I'd like to see the finished product.
Mike