Is there a power button on the PC get the thing to turn on, and if so, does that work?
The way PCs generally work is that when power is applied, there's a chip called the SuperIO which waits for the power to stabilize, then loads some settings from a chip called the CMOS. These settings can include things like whether to power up the rest of the system immediately or to wait for a keypress or some other event. Generally those settings are configurable in the BIOS. I'm curious as to whether your BIOS is configured for auto boot up. You might try configuring it to stay off, and then using a power button to turn it on.