Win2VNC

This program will let you use two screens on two different computers as if they were connected to the same computer.

It even works between different operating systems.
If you have two or more computers on your desk and you are tired of having several keyboards and mice around to control them, this is the program for you. While running this program, you can move the mouse pointer beyond the right or left edge of your primary display and the pointer will appear on the other computer screen.