i've been doing that for eons! .. hahaha ... got a few ways ... try
www.youtubex.com
or
there's a videodownloader add-on for mozilla,
a flash-get add-on for mozilla and
un-plug for mozilla ...
all those plug-ins for mozilla will do that, but for ie users, just use youtubex ... however the file saved is a *.flv file ... so u may have to convert to mp4 or avi before u can view it on PC .. can use this prog called SUPER (just google super video converter, it's by erightsoft) or 3gp converter ...