How to use PRNT SCRN to capture my window?

Hi TechyV users,
I want to prepare some user friendly instruction for kids with the basic windows Operating system options.
So to do this, I can take the screen shots.
But the problem is, when I press "PrtSc" button and paste it to Ms Paint program, it will copy the whole screen with all opened windows but i need to copy only the particular window.
So is there any easy way to do so?
