MSDN says:
The InitiateSystemShutdown function displays a dialog box that notifies the user that the system is shutting down.
During the InitiateSystemShutdown time-out period, the AbortSystemShutdown function can prevent the system from shutting down.
Also, it's only for NT, and, your program must have a certain privilege (from what I remember, setting privileges on NT is more trouble than it's worth).
Even hooks (like denniswrenn suggested) aren't "allowed" to disable shutting Windows down.
Besides, are you sure you don't want Windows to ever shut down?