Quickly googling shows this: http://www.wikihow.com/Force-a-Blue-Screen-in-Windows
I'd then suggest writing some sort of listener that does that in the background whenever you send it a command. If you are on the same network as it then just write a quick server on an obscure port that runs as an administrator in the background and then forces the BSOD whenever it receives a command from you.

EDIT: Looks the same as the first method, but I didn't look too closely: http://www.instructables.com/id/Display-the-BSoD-with-3-keystrokes/

EDIT2: If you don't want to force a BSOD, this is good for shutting it off. Same stuff from before applies https://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/shutdown.mspx?mfr=true

First of all, can I then command it without the user noticing? Second, I'm confused by talk of servers, ports, and writing - I'd need a lot of walking through that.

Sidenote, can anyone help me write a special hard-reset or BSOD Windows Vista x86 driver? That might work well.

If you set up the listener right, then yes you could do it in a way that the user wouldn't notice.

If you wrote a simple server application that all it does is listen on a port, so something like this would work (node.js)
require('http').createServer(function (req, res){
  require('child_process').exec('<command to run>');
}).listen(9615);
All you would have to do is figure out what the IP is on the network and then just load http://<machines_ip>:9615 in your web browser (as long as the machine you are on is on the same network) to cause the BSOD or shutdown.


EDIT: So, as it was pointed out to me on IRC. I really shouldn't be helping with this kind of thing. The topic is locked because, well the reasons you want to know this are kinda nefarious.
Sorry about that!