Problem with adobe photoshop 7.0

Hey everybody..I'm new here but have been watching for a while. I have a problem with my adobe photoshop 7.0, I looked through the memories FAQ section but couldn't find the answer. I was hoping someone could help. I have two computers, each with photoshop on them, I can crop images on one computer but the other, when ever I try to use the rectangular marquee tool and select an area of an image, the edges are always curved. Also, when I try to crop something a little box pops up saying "Warning: No pixels are more than 50% selected. The selection edges will not be visible". Does anybody know what that means and/or how to fix it because its really annoying! Is it something I'm doing or the sofeware's fault? I have to use my other computer, which is in a very inconvieiant spot, for bases now!