I want the screen to be blank during startup, and only turn on by a ::SendMessage(HWND_BROADCAST, WM_SYSCOMMAND, SC_MONITORPOWER, (LPARAM) -1); when my watchdog service starts the multimedia application which is the only thing that will be running on that computer. So the last parameter would be 1 or 2, but how to send this at earliest possible time during startup, or better yet, start in the low power or off state?

Not going to happen. The Video Card screen, the bios (cpu, memory count) screen and the device-list screen aren't handled by the OS. You could write a boot-time (non winapi based) program (álá Grub) to make your monitor do something, but that's not going to be easy and Windows itself will probably reset whatever you do during load.

Some motherboards (I know the new ASUS ones) allow you to put a custom splash screen up during the bios boot you could have a blank screen for that and then uses a "boot screen replacer" for windows and you will get a sort of result, messy though.

What screens are you driving? If you are using commercial displays that have RS-232 communications you can get your code to turn them on and off.

Yeah I have Aquos and wrote code for the RS-232, just that for the first couple months I won't be able to put in serial port expansion cards in all of the computers.

I'll just have to make the desktop black and replace the Windows splash screen, living with the BIOS startup messages.