With the screen showing press ALT-Print Screen. The open 'Paint', and 'paste' the saved screen in, expanding the image if needed. Save this if you want all that is showing
If you want only a part then use paint tools to cut out the relevant part. Open a new instance and paste the cut out part to it. You can then save that to its own file.
There are freeware applications which make this easier but this is fine for occasional use.