If you have Win7 then, as has been mentioned, there is the Snipping Tool. The quickest way to get to it, if you have never used it before, is to just click on the Start (Windows) button, in the lower left corner of your desktop, and type in "snipping tool" (you may not need to type all that, as soon as you start typing that, Windows will narrow down your possibilities) and click on it.
Once started you will see options to use one of the four ways of selection your capture: Free form (where you drag a line around the screen), Rectangular (where you select diagonal corners of a rectangle), Window (where you select the window, but if it is not fully exposed - some is overlaid - then the window dimensions are captured), or Full screen.
After you have made you choice of the snip, and, if needed, the area, you will be given to save the selection as HTML, PNG, GIF or JPG. There are reasons you might select one format over the other, and if you need direction there then ask in another posting - don't clutter up this thread - it will get busy.
Other screen capture tools abound in Windows. The screen capture built into all versions of windows is limited, but perhaps more useful than you realize. Sure just pressing the [PrtScn] button on the keyboard doesn't quite do what you expect - since it just really puts the screen into a memory buffer area, and it is up to you to then paste that capture into the program of your choice, but that's not too bad, just a bit cumbersome.
If you paste that capture into the built-in Windows program Paint, you can see what it is you really captured where you can then crop and edit it down from there, and finally save it to a graphic format you want (different versions of Windows gives you different formats).
If it isn't a full-screen you want to capture, perhaps it is just a window, then just hold down the [Alt] button while you press the [PrtScn] button. That will capture just the active (top-most) window.
And you still paste the temporary screen buffer area into the program of your choice, like Paint or whatever.
Sure, there are other programs around, and a lot of free ones, that will more automate the actions of what you capture and paste into the program of your choosing, but you didn't ask that. All you asked for methods of capturing some portion less than your whole screen.
Ask another question and perhaps we'll know more of what you want.