Luca explains how to boot your PC with a Wake on Lan command sent from an Arduino. Setup the WOL feature on your PC, then use the sendWOL() command to send a magic packet via and ENC28J60 ethernet chip:
The wake command is issued sending on the network a specific packet, called Magic Packet. This packet is receved by all the devices connected to local network because it presents – as destination MAC address – the broadcast address (FF:FF:FF:FF:FF:FF). It includes six bytes valued FF, followed by the target PC’s MAC address repeated 16 times.
Or you can have your micro simply actuate the power button to come out of sleep, and it doesn’t need an ethernet to do it.
Or you can plug the micro into the WOL header on the motherboard.