Only thing i can think of is, seeing as you say "almost the slowest setting" you must be adjusting the slider bar, this is part of the intellipoint software, if the intellipoint software is not installed the wheel tab page looks like this
you can then select number of lines to scroll for each wheel notch moved
Could try uninstalling intellipoint??
Not 100% sure just trying to suggest something, you can always re-install later (in fact i think windows update will prob try and install automatically when you reboot)