What version of Linux?
Is there an xorg.conf file (/etc/X11/xorg.conf)? If so, what mouse driver is specified? There will be an entry something like this:
Code:
Section "InputDevice"
Identifier "Mouse0"
Driver "........"
Option "Protocol" "auto"
Option "Device" "/dev/input/mice"
Option "ZAxisMapping" "4 5 6 7"
EndSection