Are you pasting them in Paint? Make sure your not using a tool like the eye dropper, Text tool or anything else. Most computers will only paste it if your using the square marquee tool. (The little box that looks like a square with portions cut out).
And just to clarify, your pressing the "PrtSc" button? And your also hitting Cntrl+V? Or you can right click and paste. If this doesn't work try downloading Swiftkit, using the PrtSc button, then there will be a tab at the top right of the program that looks like a picture frame, click that and it should show the screenshot and you can save it.
And if that doesn't work make sure the key actually works, you might have to take it off as there could be some sort of gunk in there.