How do you stop the computers Monitor/Screensaver turning on?

What version of Windows?

>>how do I stop the monitor turning on while I'm doing something.
Well don't touch the mouse or keyboard

If you really want to do that, use the "System Standby"
set your screen saver to 1 minute
set your turn off monitor for 2 minutes
set your system standby for 3 minutes
With that, nothing to worry, pressing the power again would start the computer again
And use also the hibernate.

There's no need to programmatically do that,  unless your building for other users for an application with that option.