It's a mouse fix that professional gamers use for accurate mouse sensitivity.
One of the Cyberathlete Professional League's members' have made this simple
registry change that will disable any mouse acceleration made by Windows XP. Be warned: it will feel akward and slow at first. This is true. By removing
acceleration, your mouse pointer, (crosshair perhaps), will accurately display
the movement that you make on your mouse. Therefore, making a very-sensitive mouse
movement. It may not have any effects out-of-game, but it is VERY noticeable ingame. It maybe wierd at first, but hell, once you get used to it, it may improve your aiming skills.