There are like millions of guides that let you download youtube videos to your PC. But this one is not only the easiest, but also most feature-full. You can opt on the file Format for saving Files: MP$, FLV, 3GP and even HD videos.
Follow Method 1 or 2:
Method 1. (requires Firefox)
If you use Firefox, get this plugin, and install. Next time you open a video on youtube, you will get option to save it in different formats on the right sidebar.
(See Screenshot on Right.)
Method 2. Direct Download
You can also checkout our Recommended Add-ons for Firefox.