Mouse on laptops

I have a Compaq Evo N115 laptop with a Synaptics touchpad (
http://compass.com/synaptics/ ). I have been using the unstable version of the Linux kernel for some time (now I'm using version 2.5.68).

My problem comes with the external PS/2 mouse. When I plug in into the PS/2 port the mouse wheel (or even, if I plug it before boot) doesn't work. I think, the problem comes because PS/2 port is shared with the synaptics touchpad, the kernel detects that the mouse type is a Synaptics and It handles like a normal 3 button mouse without wheel. The mouse wheel works well if I use a 2.4 kernel series (probably, because the X-window system is the one who handles the mouse wheel.). If you need some more information mail me.

Thanks for all.

P.D.: Also, it would be great if the synaptics touchpad (or any other touchpad) is disabled if other mouse is pluged into the same port.