The best way of downloading a Youtube vid to your computer is creating a bookmark pointing at
Code:
javascript:if (document.getElementById('download-youtube-video')==null && !!(document.location.href.match(/http:\/\/[a-zA-Z\.]*youtube\.com\/watch/))) {var yt_mp4_path='http://www.youtube.com/get_video?fmt=18&video_id='+swfArgs['video_id']+'&t='+swfArgs['t']; var div_embed=document.getElementById('watch-embed-div');div_embed.innerHTML=div_embed.innerHTML+'<br /> <span id=\'download-youtube-video\'><a href=\''+yt_mp4_path+'\'>Download as MP4</a> (click and save)</span>';}void(0);
That will add a download link at the bottom of the description box, if clicked while you're on the video site.