There's a few ways of doing this you could create a shortcut with the following:
shutdown -s -f
This would create a shortcut that you could click on, you could also set it to reboot by replacing -s with a -r
If you want your system to shutdown automatically at certain time you would need to write a batch file (just make the extension .bat or .cmd, i.e. shutdown.bat) type the command listed above in the batch file and save it to you Windows or Winnt directory. Go to a DOS prompt by typing cmd on the run command box or from Start\Programs\Accessories folder select Command Prompt. At the command prompt type the following:
AT 23:00 /every:m,t,w,th,f,s,su shutdown.cmd
You could change the time to what ever you want it to be, it needs to be in military format, or if your looking for a specific date it could also be changed. Hope this helps.
Or you could create a scheduled task that shuts it down whever you want. Windows XP recognizes "shutdown" as a valid command so you would simply tell the task to perform "shutdown" like a run command and it will work. I believe "restart" and "standby" are valid commands too.