There are a couple easy way to do it. Check these out:http://howto.wired.com/wiki/Save_YouTube_Videos_To_Your_Hard_Drive
I tried the add on for the firefox browser and when you bring up the youtube video there is a arrow pointing down next to
FLV,MP4 or 3GP. Click on one of these and you can select a folder on your hard drive to save it to. MP4 saves as a quicktime video.