I've just had a little look round and it is certainly a very interesting topic. One thing that you might want to give a try and looks like something that would be easy to start off with is using a version of Linux bootable from a CD and booting up each PC. All of the PCs can be networked together using standard Ethernet connections and a router. And there you pretty much have an instant cluster, it obviously won't be capable of as much as a cluster with a dedicated OS and high performance networking performance, but it is certainly a start before you move onto bigger and better things. If you want to move on further, using Linux is your best bet.
This is the page with information about CD based clusters.