just googled your question, this is the first result I got, it seems to work, try it
If You Had Firefox, you can use it's downloadhelper plugins to download videos from almost any site, that includes youtube (and joost, I just tried to download videos from there with firefox and downloadhelper, and it works).
If you don't had them, then
To Download Firefox, click on the link:http://www.mozilla.com/en-US/firefox/
To Download downloadhelper plugin for firefox, click on this link:https://addons.mozilla.org/en-US/firefox
To Learn how to use downloadhelper, here's a youtube video tutorial if you need it:http://www.youtube.com/watch?v=CRzOej3io