I have an ECS motherboard that doesn't automatically restart upon power fail. The machine is in my attic, and provides (MP3) music 24/7 to my bathroom. I got tired of going up to the attic to restart it after every power flicker.

I use a 12F675 powered from the 5V standby of the power supply. One PIC output pin is tied to one of the "PowerButton" pins. After power up, I wait for 3 seconds (for the power supply voltage to stabilize) and then write a 1 to an output pin and set it as an output for 300ms, then I set it back as an input. This simulates an open-collector driver (so that the ordinary power button still works). The entire circuit consists of the PIC and a 10uF cap across the power leads.

Works great.

My motherboard needed a logic "high" on the pin to start it. Check your motherboard. It may need a logic "low". In that case, just write a "0" to the PIC pin before you set it as an output.