One technically feasible way to go about it would be to create a script on a particular host, and make sure all of the other hosts had the primary host's ssh key in their authorized_hosts files -- a simple foreach loop could iterate through your serverlist issuing the 'shutdown -h now' command from root.
However, a) having root be able to SSH in is a very bad idea, b) it's a rather complex set up that is far more confusing to debug if something should fail than other solutions, yet c) it's a good way to learn quite a bit about the various elements that would have to interact for such a script to happen. Oh, this all assumes you log in via putty (or some other measure of CLI) to the first host to issue said command.
The more specifics (how are these computers networked? Do they need to go down with any particular regularity, on a schedule, or randomly? et cetera) you give, the better and more narrowly-defined (to your situation) answers we can provide.