You can schedule a shutdown at a given time with XP pro. Probably XP home also. Note that you also configure this for a specified "idle time" - although I would be leery of this, depending on what is running on the PC (it may never be "idle" enough for the system to shout it down).
Trial and error is, as usual, your friend!
Use the Schedule Task GUI and the task you want is (unsurprisingly) SHUTDOWN /s.
You can see a list of switches and parameters by typing "shutdown" at the command line. And no, it won't shut down straight away - you have to have the /s appended!
SD