Did you check your XF86Config-4 if using Xfree 4x, if so it should read something like this :
Section "InputDevice"
Identifier "Mouse1"
Driver "mouse"
Option "Protocol" "IMPS/2"
Option "Device" "/dev/usbmouse"
Option "ZAxisMapping" "4 5"
EndSection
Assuming your USB mouse is considered Mouse 1.
PS If using XFree 3x then it will be /etc/X11/XF86Config
Good Luck
