Emulate a KVM switch using software
Avi Dardik has a quick guide on how to set up a secondary PC using VNC (for multi-platform support) to act like a KVM switch and allow you to switch between active full-screen sessions on your host machine and a client machine at the tap of a hot-key. Although many may argue that using VNC software for this purpose is a pretty ugly hack, the reality is that this scenario is extremely common, and very fast on a local network.
The only thing I would add is that if I was going to set this up with strictly Windows PCs, I'd substitute Remote Desktop for VNC. Remote Desktop is simply so much faster and more efficient between Windows PCs that there's no reason not to. That is, of course, if you have XP Pro or a flavor of Windows Server for your "client" machine. XP Home won't allow you to make a Remote Desktop connection to it unprompted.
[image courtesy of Wikipedia]