I agree, the shutdown command will work... depending on how locked down the operating system is. If I remember correctly, there's a setting in the Local Computer Policy that can restrict everyone (even Admins) from using that command. My network at work is locked down like that (it has to be if you want to pass a certain government agency's inspection). I use my code to remotely shutdown/reboot/log off computers on my network from a web interface. This will work if a certain port on the computer isn't blocked (snmp, I think. We're not allowed to have host based firewalls).

And yes, you can exec an external command from almost any programming language.