#Adapt the following to your xorg.conf
Section "InputDevice"
        Identifier "usbmouse"
        Driver "mouse"
        Option "Device" "/dev/input/mice"
        Option "Protocol" "IMPS/2"
        Option "ZAxisMapping" "4 5"
EndSection
Section "ServerLayout"
	InputDevice "usbmouse" "SendCoreEvents"
(...)
EndSection
#I have a serial mouse as CorePointer, just to make stuff clear.