There might be a lot of options but I can think of two just off the top of my head: VNC or a KVM switch.
VNC - is a program that you install on all the PCs and then from almost any computer(if its set up correctly) can access the PCs. This site
has tons of info. There are a few flavors of VNCs out there.
KVM switch - is a small box that you plug your mouse, keyboard and monitor into and has a number of wire bundles to plug into the PCs you want to switch to. Here
is a picture of a 2 port KVM switch.
I am not sure how "away" you to want to put the boxes but either will do the job. VNC will allow you to get into the boxes over the internet, if you set it all up for that. The KVM switch gives you an actual physical connection to each of the boxes but they can only be put "away" as far as the wires.