I'm assuming that Win 8 is not different from Win 7 in this aspect, so:
On your desktop click on the <Start> button, select <All Programs>, navigate and open the <Accessories> folder. In this folder you'll find a Program called <Snipping Tool>.
Use that to make your screen shot. First open Lr to the Grid View where you see the grayed out photos. Then start the <Snipping Tool>.
When you start the <Snipping Tool> your screen will go light grey, and your cursor will be like a cross. Then just drag a square around that part of the Lr window that you want to show in the screen shot. When you are done, still in the Snipping Tool, go >File >Save as. Save it wherever you want but where you'll find it again.