How to: Movie in Movie?

Looking for some software that enables inserting video into an existing video.
I don't mean simply a box in the screen. I'm interested in superimposing a cropped moving image onto a video clip. Say, a different head on a body.

I've seen this done but don't know how it was accomplished.

I think this is what people use After Effects for. Maybe Motion and Shake too. This isn't a low end effect but with a lot of work, it might be doable in Final Cut.

Thanks for the replies. I have an older copy of AfterEffects I've never used - so I'll try that and also
look into the others.

Final cut and final cut express allow you to do this

Thanks Del. Express may fit my budget. I'll check it out.

Del said:

Final cut and final cut express allow you to do this

Click to expand...

They allow you to overlap video, but it would be difficult to create the transparency needed for the head shot, excepting a green screen type effect.