Windows 7 provides you with a tool called the Snipping Tool, with the help of which you could actually select a portion of the screen whose screenshot is to be taken. This is very helpful since you are now freed of the hassle of 'Prnt Scrn'-ing the entire screen and cropping the required portion out. You can start the Snipping Tool from the start menu.

This screencast below will make it clear:

If you can`t view the above screencast in your browser, go to it`s direct link http://blip.tv/file/1726366/