|Is this your situation ?|
What you have sounds like you have a "virtual desktop" or similar feature. The virtual screen size (resolution - width x height) is larger than the physical screen size (resolution).
E.g. a 1280 x1024 virtual display on a 1024 x 768 or 800 x 600 display.
The display stays stationary when the mouse cursor is not near the edges of the display, but when you move the mouse cursor to the edge of the screen, the whole display moves in the same direction you moved the mouse to.
Power Toys for XP has a feature that does that.
If Power Toys have been installed, there's probably an icon for that in Control Panel - Classic view.
If this is happening on an Apple computer, it has a Zoom feature.
See response 3:
If neither of those apply, you need to tell us the make and model of your computer.