Chicken and egg, eh?
Seriously, though, what you're trying to do isn't "supposed" to be possible because UAC (though disclaimed by Microsoft as not being a formal security boundary) is a security boundary of sorts.
In a domain environment you should be controlling User Account Control from Group Policy (the "User Account Control: Run all administrators in Admin Approval Mode" setting under "Security Settings\Local Policies\Security Options" in the computer section of Group Policy is what you're probably looking for).
If you're in a non-domain environment then you'll need to run the script in an already-elevated context. As @womble says in his comment you'll need to bypass UAC in order to disable UAC without invoking UAC. A computer startup script would be one place that you could run this. To install it as a startup script, though, you'll have to be elevated. You're back to the chicken and egg, then.