Tom
When the Zoom Tool is active, up on your options bar, (upper left, just under the menu bar) you should have two little windows that show two little magnifying glass icons, one positive, one negative. Simply click on the positive one. And when the Zoom Tool is set to positive, hitting the Alt key will switch it temporarily to negative.
- Unless you have an ancient version of Photoshop (before Photoshop 5 - before the options bar appeared)...in which case, I can't remember where it is to change it.
David