So what you did, if I've got this right, was have a mouse connected to the USB port but then switched the machine off and then powered up with it connected to the PS/2 port instead.
You will probably need to tell Windows to detect the new hardware and learn to navigate using the SPACE, RETURN and TAB keys to do the install.
Boot up, press windows key and Break at the same time, right arrow twice times and then ALT and H together. You should then be able to do the hardware scan.
Another thing - make sure the connector is all the way in (as some don't fit too well).