First, your motherboard and NIC have to support Wake-On-Lan. After it is enabled you can send the "magical packet" to turn the PC on and do whatever. I use this for running ghost after hours.
Here is a link to a place that has a utility you can use to test your wake on lan settings or to wake up PC's remotely:
http://www.solarwinds.net/Tools/Netw...g/Wake_On_Lan/