Ed,
That's a pretty basic Windows function and not a lot can go wrong. Plus, any photo editing software should be able to accept a paste from the printscreen function of Windows.
Is it possible that you have one of those MultiMedia keyboards where there is an "F Lock" key that toggles the functions of the F keys from their usual functions (F1, F2, etc.) to Internet functions (F1=Help, F2=Undo, F3=Redo, etc.)?
PrtScn isn't an F key, but the F Lock toggle key makes it work differently as well.
If you do have a MultiMedia keyboard, toggle the F Lock key and try it again.