"How would I find such a culprit without having to install Windows again?"
Since Microsoft doesn't provide drivers for most video cards or decribe many monitors, installing the OS and more from Microsoft is only the beginning of this OS install. Many only learn this by others telling them so...
Fetch the latest video card drivers and monitor.inf and that should help. Remember, "driver update" buttons don't do this for you.
I suspect that something in my Registry is preventing my monitor from turning off. I have played with every setting in the "Monitor Power Settings" and it has had no effect on turning off the monitor.
Is there any other "hidden" place to change the monitor shut off setting other than Control Panel/Power Options?
How would I find such a culprit without having to install Windows again?