1.Open You Tube and search for a video that you want to download. Once you've clicked on the video, click again the HD button.
2.Once the page refreshes,Click on Windows and scroll down and click on Activity. Once the activity bar opens click the down arrow and search for the address that is streaming in mb.
3.Double click that url and your download should start. Generally youtube videos will be downloaded in .flv meaning it needs to be played in a flash video player, however what I've found out is that if you download the file in HD it will download as a quicktime file (.mp4).
enjoy
