Does anyone know where I can get a cable and program for a PC that will get access to the on-board computer, especially the fault codes?

Not offhand, just get a generic OBD-II scan tool. However, you're in luck in that the 2000 was the only Xterra model year that had a self-diagnostic feature. There's a tiny screw on the passenger side of the ECU that you can turn to cause the check-engine light to flash out the code(s) in the system. I can elaborate if desired.