Now, is this happening on only a single computer? Have tried re-installing Windows?

I have no problems shutting down and loggin off users from remote computers. As a matter of fact, we are using these KiXtart scripts within a custom client-server data acquisition system where the controller client has to shutdown the server as the server has no input devices (keyboard, mouse, touchscreen) except custom data acquisition electronics, sensors, and electrodes.
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There are two types of vessels, submarines and targets.