In order for printscreen to work, you must first turn down the hardware acceleration. Especially if you're using a DVD. The way it was explained to me is that DVDs and some videos play on a different layer of the screen because of the quality, so when you printscreen normally it just takes a cap of the base layer and so the cap you make winds up having a black box or a window where you can see the DVD window through it. It's weird, I know.

Turning down the hardware acceleration makes it so that the DVD plays on the base layer. It plays a little choppier, but it caps. To turn down the hardware acceleration, go to tools - options - performance - advanced, see that bar? Pull the thing all the way down on it. Now hit okay and press printscreen on the part you want to cap. Then ctrl + v it into windows paint or whatever you're using to look at your caps. It should work, if not, try looking around the memories of icon_tutorial, they might explain it a little better.