Synergy

Synergy lets you easily share a mouse and keyboard between multiple computers on your desk, it's free and open source. Just move your mouse cursor off the edge of one computer screen and into another. You can even share all your clipboards. All you need is a network connection. Synergy is cross-platform (works on Windows, Mac OS X and Linux).

Download Synergy 1.4.10 for Windows 64-bit here.

To get it up and running, you need to click the "configure server" button on the computer where you have the mouse and keyboard you want to share, choose the placement of the additional monitor here, and choose the "hostname" of the computer that will connect to the server.