If you are in xp, save the youtube file to your computer with videodl.org or a similar service, open the file in notepad, and you should find a link to the originally posted avi or mpg or whatever the poster uploaded it as. Enter that in firefox or IE, and download.
I say you Should find a link in the notepad file, because I do this with google vids, and I am assuming it works the same way with youtube. Just a thought.