It is typically sent as a UDP datagram to port 0, 7 or 9
synch stream: 6 bytes of FFh
16 times the repetition of the MAC address
e.g.
broadcom netlink fast ethernet: 00-1a-a0-fc-94-62
intel pro/100 ve: 00-00-e2-90-02-27
some methods to see if you have WoL enabled... (although some may seem to refer to wake from hibernate/standby instead of off)
in the Bios look for WakeOnLan or WoL from in winxp device manager right click on the NIC -> properties -> advanced -> wake up mode
from cmd.exe: powercfg -devicequery wake_armed (press Enter) or powercfg /devicequery wake_from_any
easiest way, download a ready made windows utility: http://gammadyne.com/cmdline.htm