Similar to what @Jeff Tullin says, if you’re using a built-in MacBook keyboard or an external Apple keyboard – i.e., one without the physical “PrtScn” button like a traditional Windows keyboard would have – then you can press this 3-key combination on your keyboard:
Command (Windows key under Bootcamp) + Control + o
This will show the Windows on-screen virtual keyboard. From there, you can use your mouse to click its “PrtScn” key. At that point, as Jeff noted, Windows will have taken a screenshot of your screen and will have copied it to your clipboard. Then you can open the Paint program and paste in there to save your screenshot as a PNG or JPG image file, for example.