I'm going to assume that "versioni 7.3" means Red Hat 7.3, right?
Open up your /etc/X11/XF86config file, and look for the section that says "InputDevice", with "Mouse1" as the identifier. It will probably look something like this:
Code:
Section "InputDevice"
Identifier "Mouse1"
Driver "mouse"
Option "Protocol" "PS/2"
Option "Device" "/dev/psaux"
Change the "PS/2" to read "IMPS/2" and add this line:
Code:
Option "ZAxisMapping" "4 5"
That should do it. Save your changes and restart X.