Also, by pressing Alt + Prt SC, you will only get a screenshot of the window that has focus which will ultimately give you a smaller file size.
The snipping tool provided in Windows Vista + Windows 7 (http://windows.microsoft.com/en-US/windows-vista/Use-Snipping-Tool-to-capture-screen-shots) allow you to capture selected areas of the screen, and Windows XP can use a free tool called, “Snippy” which is free and available here: http://www.bhelpuri.net/Snippy/