Not quite sure if you are asking to shutdown just a client workstation or if this XP workstation is sitting in your server room running some simple application and you want to be able to remotely shut it down.
For clients.....you can do it with a power shell script, see link below
http://www.infotechguyz.com/cmd/powershellremoteshutdown.html
Or, if its a box sitting in your server room, then you can use the APC Switched PDU with the web GUI to turn on/off ports.
http://www.apc.com/products/resource/include/techspec_index.cfm?base_sku=AP7900