A screen-capture utility comes built into Windows that allows you to capture the current screen or window. All you have to do is press the 'PrtSc' button (aka 'Print Screen') to capture the current screen, or press the 'Alt+PrtSc' buttons to capture the current window to the clipboard.

Then load an image-editing program (such as: Windows Paint) to view, edit, or save the captured image. Press Ctrl+V to paste the image into it.