what does your xorg.conf say ?
it shoule be somethig like this ...
IMPS/2 and ZAxis options are important...
Code:
Section "InputDevice"
Identifier "Mouse1"
Driver "mouse"
Option "Protocol" "IMPS/2"
Option "Device" "/dev/input/mice"
Option "Emulate3Buttons"
Option "ZAxisMapping" "4 5"
EndSection
i hope this helps