There is already a system command line tool to trigger a restart / halt / logoff on a local or remote machine.
For example, if you want to halt a remote computer named TESTCOMPUTER, the command line is the following:
shutdown.exe -m \\TESTCOMPUTER -s -f -t 0
Of course, this command line will have to be executed by an administrative account.
Using a silverlight app for this specific purpose is nonsense, IMHO.