Screen capping vids, why the eff doesnt it work? Help! :(

well i want to take a small part out of a video but i cant do it. when i press prtscr and paste it to paint, where the video shot shoud be is black and when i move the shot over the video, it shows the video there under it as i move around. please help and tell me whats going on. im trying to take out a few frames for use in an animated spray in counterstrike, and if i cant solve this i dunno how i will do that.