Old Vaio Mouse Problem

I have an old Vaio Laptop PCG-F650 and most of the time use an
external compaq PS/2 mouse through the 9-pin connector in the back
instead of the touch pad.

The external mouse works fine, but whenever I boot the computer anew,
the mouse freaks out only by touching it; it's all over the screen.
Then, I have to unplug the mouse, touch the mouse pad a couple of
times; the pointer eventually stops going erratic. After this
procedure, I can plug the mouse back in through the 9-pin and the
mouse works fine. Now, I have to repeat this procedure everytime I
shut-down windows. I tried disabling the touch pad, but this does not
help.

Anyone has an idea? I'd appreciate any advice.

Thanks.