Run a script on Windows startup without a user logged on

This is a Windows 2003 (or later) machine that gets switched on every morning, but no one logs on until some hours later.

I want to use the time in between to run a backup script c:\script\backup.cmd

How can I start this unattended after the machine has come up?

I tried 2 registry keys, but this resulted in the script being run after a user logs on (which is too late):

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\RunServices

In the end I used Windows TaskScheduler, who has such an option, but I was wondering if there is another possibility?