WINDOWS 7 or VISTA
Copy any portion of your screen to a file!
By using the Snipping Tool, you can select the area that you wish to save and copy it to the Clipboard or to a JPG file.

To open the Snipping Tool, from the Start Menu, go to All Programs | Accessories (or just type "snip" in the Start Menu search box).

BTW, in any version of Windows you can copy a screen shot to the Clipboard by pressing the PrtSc (Print Screen) key.  A copy of the active window is copied to the clipboard by pressing Alt-PrtSc.