If you should wish to download a video from YouTube for later viewing, here is a simple little program...(I tested it...it worked fine on my 1st try) It puts a little icon in your browser's titlebar, which you click to get the options banner...then paste the video address in its space and zooom...

http://www.remlapsoftware.com/

I used VLC Media Player to play the file, (it plays a lot of formats)

or here is a stand-alone Flash (.flv) player
RivaPlayer (I have NOT tried it)