Windows includes a print screen key combination that allow you to capture a single window on your screen, in addition to one that captures the entire screen. When you use these commands, the resulting image is stored into your clipboard for pasting into applications. You can copy and paste a window-sized part of your screen into any document that expects image input, including Microsoft Paint, which is built into all Windows installations.
Place the window you wish to capture into the active focus by clicking it on the task bar if it is not already in the foreground.
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Press and hold "ALT" and then press "Print Screen" on your keyboard. The key may be abbreviated "Prnt Scrn."
Click "Start" and type "mspaint.exe." Press "Enter."
Press "CTRL" and then press "V" to paste the image of the window into the screen.
You can use any program that accepts image input in the active window to paste your image.