This free, cross platform, open source software allows you use the keyboard/mouse from one PC, and control both PCs (the mouse just leaves one monitor and enters the other one) with it. It acts as a keyboard/mouse switch (not video switch: both PCs need to have a monitor), which is very convenient for us tablet PC users.
Just connect the two PCs in network, start the Synergy client (PC whose keyboard/mouse you will use; configure the positions of the monitors), start the server and you can control both PCs with the same keyboard/mouse. (you are not limited to 2 PCs, nor do they need to run the same OS)
It also shares the clipboars (copy text on one PC, paste it in the other) and synchronizes screensavers.
I'm not affiliated with them, but I really find the software useful.
Hope some of you find use for it as well!