An alternative is to use chrome and its fantastic download video function.
Download
Google Chrome, as it can download YouTube videos, it is 10 times faster than IE, and the flash player will install easier. Next, copy this script:
Code:
javascript:window.location.href = 'http://youtube.com/get_video?video_id=' + swfArgs['video_id'] + "&l=" + swfArgs['l'] + "&sk=" + swfArgs['sk'] + '&fmt_map' + swfArgs['fmt_map'] + '&t=' + swfArgs['t'];
Once you've download Chrome and copied the script, go Ctrl+Shift+B. Then Organise> add page and paste in the script, and save it as "Download Video".
There you go! When you are viewing a You Tube video, click download video and the download will begin.