Open a program called Snipping Tool (type Snipping in the "Start Search" area of the start menu).
Click on the arrow next to the "NEW" Button, then click "Full Screen Snip"
Click on the Save Button (or File -> Save As), and choose a location to save the file, Example: Desktop
Windows 8, Windows 10
Use The Keyboard Shortcut - Windows + PrtScn
If you want to take a screenshot of the whole screen and save it as a file on the hard drive, without using any other tools, then press Windows + PrtScn on your keyboard. Windows 8.1 and Windows 10 will store the screenshot in the Pictures library, under Screenshots.
Windows XP
Ensure the screenshot you wish to take is visible on your screen and press the Print Screen (PrtScrn) button on your keyboard.
Open Microsoft Word or Microsoft Paint
Paste the image into the program by holding Ctrl and pressing V, or on the menu Click Edit -> Paste
Click File -> Save As, change the type to PNG or JPG, and choose a location to save the file, Example: Desktop