The general consensus of the 'professional' fps players is that mouse acceleration is horrible. The reason is that the crosshair will always move a different distance depending how fast you move the mouse between point a and b even if the distance is always the same.
Believe me just because you turn of mouse acceleration on hardware/software drivers and in windows does not mean it is gone. The way to test it is to turn your sensitivity WAY down in game. Shoot a couple bullets to make a mark in the wall and flick your mouse all the way across your mousepad like you were making a 180 flick shot. Then you want to SLOWLY move it back to where the mouse started at. If it doesn't end up in the exact same spot you have mouse acceleration somewhere on your system.
GotFrag eSports - Files - CPL Mouse Fix
This turns off all acceleration through the registry in windows xp.