It's called a KVM switch (Keyboard-Video-Mouse). Depending on whether you need USB or audio switching, they run from about $15 to $100. You can compare a number of vendors at www.pricegrabber.com by typing KVM in the search box.
I have used the IOGear 2 port USB KVM for over a year - it has always worked flawlessly. It is about $50 - $70. Be sure to get a KVM that allows you to use a keyboard hotkey to make the switch.
They also have them that you can use with speakers also. I think they are referred to as KVMA (keyboard, video, mouse, audio) switches. I have been considering replacing my Raritan with one that will do audio.
Works great, couple of quick taps on the Scroll Lock key and you're right over to the other computer....sure saves on the power/heat generated by another monitor....
Don't use those cheap switch boxes, there's really no need in most cases. Just more $ that doesn't need to be spent, and more crap powered and laying around.
Ever think about remote desktop in winxp? You can access and operate as many winxppro systems as you have.
Works wonders when I need to access other systems here. Free if you're set with the home network...no lame switching (and they usually break/fail with more than average use).