Head masking effect for movies?

i wanna know how to put heads into video clips like this guy did, http://www.youtube.com/watch?v=rZBA0SKmQ... how do i do it?Head masking effect for movies?
Haha cool video.. funny as hell





anyway, i done something like this.. i overlayed a video clip over another one.. or in the case of that video they used still images of his face which is easier..





It can be done easy in Adobe Flash if you have the patience..


Im not sure of any others that can do it, but havent heard of any free ones..





If you have adobe flash say so and ill tell you how





thanks for showing me this video dude