If it's a mouse which uses a ball underneath, what you describe happens when the ball and the rollers at each side of it get clogged with debris like dust and hair picked up from the mouse-mat. Turn over the mouse, remove the little plate which allows the ball come out, take out the ball and clean it, and also clean the rollers where the ball sits (don't use any liquids to do that, except a cotton-swab moistened with a spirit-type cleaner such as cd cleaning fluid -- moistened, not wet through.
Better still, if it is a ball-mouse, just bin it and buy yourself an optical one which doesn't use a ball. You'll notice the difference as soon as you use it.
Edited by pip22, 07 February 2007 - 03:32 AM.