In which case I'd go back to the beginning, convert each video into the same format, and same aspect ratio.
I'd pick one of the computer friendly formats, such as WMV to convert to.
Pick your convertor carefully, they don't all do as good a job as each other.
My choices would be Super or freemake.
Edit each clip one by one, adding them and editing them, saving a movie each time as well (so if there is a problem you can detect which one is causing it) and when finished save out ready to go to youtube.
Here are some links to useful conversion apps.http://www.myvideoproblems.com/WebPages ... rammes.htm