Control panel > mouse
adjust the sensitivity in there so it is most like your old mouse.
Anyways, laser mice will be a better adjustment as ball type tend to get stuck or jumpy.
No, no, no, no, no! The only thing you want to do in there is uncheck "enhance pointer precision", as that adds mouse acceleration, which will screw up your aim. If you adjust the mouse speed settings in control panel, windows will mess with the mouse movement in software (so the movement required to move 2 pixels may only move 1 sometimes, but other times, 2). You're also cheating yourself out of in-game precision. You want the control panel setting to be exactly in the middle so windows doesn't speed up or slow down the cursor movement, then adjust the sensitivity setting in-game.