Sounds like you clicked the middle mouse button and activated scrolling like here http://www.techsupportforum.com/forums/f25/mouse-autoscrolling-how-to-slow-it-down-113822.html. Click the middle button again.
OR maybe you set your display resolution to higher than your monitor can display and you are now scrolling around the extra area. Check your Desktop resolution and compare that to the monitor native resolution, both should be the same.