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: [via]
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.
Boot your PC with the Arduino using the Wake on Lan command – [Link]