By default, something most win7 users might not know, is that your computer came with something called the "snipping tool" that allows you to take a Pre-Defined rectangle of your screen, and turn it into a PNG/JPEG picture.
After selecting your Region, You will see a screen with your picture, and a pen/highlighter/eraser, Which could be used to block out certain things, or circle em, ETC.
The tool is located (upon clean install) in the start menu, but can be reached by going to your "all programs" or by typing in "Snip" and hitting enter, when the start menu is open.
It's a pretty simple thing most of you know, if you do know that, don't post that here, please

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Also, if their are any programs that can do this, for other OS's, Post them here and ill add it with credit