XP uses the drivers for ACPI or whatever other power configuration it's using. Command.com and cmd.exe provide no such executable. The only reason a PC needs to be able to shut itself down is to update the swap file and power down some devices, and no command prompt uses a swap file or any mission-critical devices, the command has never been created.

I'm sure a programmer could create a 'boot disk' that would simply power the computer down, but I have never heard of it being done. And there may be other OSes that offer this feature, but not from Microsoft. What are you trying to accomplish with this?