I don't really know how to download the original videos, but at least you can use a trick by adding &fmt=22 or &fmt=18 to the youtube URL, so that you can download HD or HQ video from Youtube.

You can download YouTube videos for free with Replay Media Catcher or freeware Ask & Record Toolbar. And you can use many other methods mentioned in our FAQ:
FAQ: How to record/download/capture/save Adobe Flash .FLV video HTTP stream from embedded player