How could I schedule it so Windows XP will shutdown automatically at a certain time?

go to the task scheduler (available from the control panel)
right click and add a new scheduled task
name the task shutdown PC
right click on the task and choose properties
in the run box enter switch to the schedule tab (along the top)
choose when you want the pc to shutdown
click okay and exit the task scheduler.
your computer should shut down when it was scheduled to.

NOTE: i have not tested this as a task, only as a desktop shortcut :)