There's a small utility called "SHUTCMD.EXE" included in the Windows NT Resource Kit that you can use to shut down or restart a local or remote computer. To shut down a computer on your network ('SERVER1' being the name of the computer you want to shutdown):
SHUTCMD \\SERVER1 /Y
To shut down a remote computer connected to the Internet ('22.214.171.124' being the IP address of the computer you want to shutdown):
SHUTCMD \\126.96.36.199 /Y
NOTE: You must first logon to the computer that you're trying to reboot.