To capture screen or to change as image file, anything that is on the screen you may press ‘Print Screen’ [PrtScn] key on your Windows keyboard and then change as image file duly pasting in Adobe Photoshop or Microsoft Paint. Instead of taking all this trouble, to capture screenshots of your desktop or any active window on the screen in only one shortcut way, install ‘Mr.Shot’ portable screen capture tool. 
@Mr.Shot there are customizable shortcut keys to capture desktop, window, click ‘N drag box and click ‘N draw freehand. After capture, enter filename in the opened window and click save to save the screenshot as image file. Image file format can also be changed.
The default shortcut keys are as follow:
- To capture a screen shot of your desktop press Ctrl + D
- To capture the windows viewed on your desktop (or active window) press Ctrl + W
- To take the screen shot of a selected box in the required place by click ‘N drag press Ctrl + R
- To select a freehand for capture press Ctrl + F (Click ‘N Draw)
To free download Mr.Shot and more free software go to en.softonic.com/