Photoshop 7 tool problem?

I'm having a weird problem with Photoshop 7, and I was wondering if anyone could help me out. I opened up the program to work on an icon, and when I tried to use the brush tool, it didn't do anything. I'd set it to a navy-blue color and was using a mouse... nothing. The funny thing is, the action showed up in the history tab - I could undo and redo it - but it didn't do anything to the image. The same thing seems to be happening with the erasor, blur/smudge/sharpen, and dodge/burn/sponge. However, it looks like fill/gradient, text, and all selection tools work as they ought to.

I've tried using the computer mouse, my tablet's pen, and its wireless mouse (my tablet is a Wacom Graphire 3), and the same thing happens with each one. I've never had this happen before, and it seems a little bizarre... any ideas on what could be wrong? I'd try re-installing the program, but the CD is floating around somewhere in my house and I'm not sure where it's ended up... (as soon as I DO find it, I'll re-install, but for now... any other ideas?)

Thanks very much.