How to use print screen to capture only a part of the screen?

I know how to use print screen to make a screen shot of one window or the hole screen, but I only want part of it can I do this? if not how can I make a picture of the hole thing and cut the unwanted part away?
Update: OK NOW THAT I HAVE WHAT I NEED, HOW DO IT GET IT BACK IN PICTURE FORM CHANGED

ITS A SLIDE
6 answers