Thanks Farrell - I've had a good look around with Device Manager. When I plug it in I see XP create a new USB device for it which is ok but nothing is showing as going worng. It's just not loading the additional USB Mouse device support.
I've tried the power-off restart as well, again to no avail (taking the mouse out, rebooting without it to check it isn't being left in Device Manager, then powering off before inserting the mouse & switching back on). When the PC is booting, I see the optical underneath the mouse light up as the USB devices get interrogated.