Sorry to hear the Aeros extensions didn't work, I really thought that was the culprit. :(
Anyway, now that we know that wasn't it, I'm wondering if it's the DPI of your mouse? Typically a DPI as low as 500 is enough for most resolutions, but just in case, what is your current DPI?
Also, of course be sure that you don't have any acceleration enabled, either through windows or your mouse software. If you use chris' fps config and the launch options that come with it, this along with m_rawinput 1 should disable windows mouse accel, but to be certain, check that in your control panel-hardware-mouse-pointer options-enhance pointer precision must be unchecked.
Sorry if that sounds dumb I'm just trying to rule out variables.