You cannot directly connect a laptop to a desktop's VGA output and expect to see the output on the laptop screen, since the VGA port on the rear of a laptop computer is an OUTPUT, not a monitor INPUT.
The only route I can think of which would enable you to use the laptop screen as a monitor for the PC would be to use a product like MaxiVista or Synergy. This enables you to have the laptop machine run as normal and connect over your network to the desktop. You can use the one keyboard and mouse, but the communication over the network enables you to use the laptop monitor as if it were an additional monitor connected to the desktop. The MaxiVista site has a good explanation:
http://www.maxivista.com and Synergy is free.
-Matt
