Open the window of which print screen you want to take.
Then press "Print Screen" key on your keboard (Next to "F12" or "Backspace")
Then Open Paint Brush (Start+run+pbrush+OK)
In Paint Brush, click edit + Paste (Ctrl + V)
Then click on "File" + "Save"(Ctrl+S)
Give path where you want to save, file name and File Format "JPEG"