Screencapping problem

Yes I looked through the memories but I didn't see this problem. I have some video files that are Core Media Player video files and I want to cap them. Someone suggested print screen so I did. But when I pasted it in adobe (or paint) there was this black square around the image itself that would NOT go away, even with cropping. It moved with the image if I moved the image. Anyone know what I could be doing wrong?