Selecting

I have PS 7, and for some reason, whenever I try to make a selection using the marquee tool, it says either, "no pixels were selected" or "No pixels were more than 50% selected. The selection edges will not be visible" when it is apparent that I AM selecting something. (It does this even when I try to select a large chunk of an image)
The "select all" option works fine.
Can anyone help me out?

(I hope it was ok to ask this here)

Thanks!