You will need I/O devices (mouse etc.) to set it up. Set up as a home network, connecting both PC's with an Ethernet CROSSOVER cable (normal patch won't work). Set up as you would a normal home network, ensuring that the old PC needs no login details to start windows. Ensure you set relevant file sharing properties etc. You should be fine without any 3rd party software, depending on O/S etc of 'old PC', this should work ok.
Hope this helps!