Shutdown multiple Windows XP machines with a Mac
Follow this tutorial to configure the windows machines: http://www.howtogeek.com/109655/how-to-remotely-shut-down-or-restart-windows-pcs/

Then install samba: http://forums.boxee.tv/showthread.php?t=38629

Then use the net command to execute the shutdown command just like from linux:
http://www.commandlinefu.com/commands/view/2330/shutdown-a-windows-machine-from-linux

If that works you can make a sh file with the command repeated for all the ip addresses of all the computers.