A simple RX TX pair would easy perform this - xbee is massive overkill.
Take a look into Home Easy hacking in the playground their sockets and switch range would allow you to do this without risking mains voltages. You might even be able to get the result you want from their off the shelf products as most BIOS's these days have an option to power-up upon power restoral which would make the PC power up when mains voltage is connected.