You Can Use The Shutdown Command In Windows XP

You can use the shutdown command, to use it click Start-Run and type
Shutdown -s -t X (where X is the time in seconds) it will open a window that
will count back from the time you specified and shut down your PC on 0

Shutdown can take time parameters as well; all you need to do is use -t

The Shutdown.exe command line command is available in Win XP to facilitate this.

An easy way to do it would be to write a one line batch file containing the following command:

shutdown.exe -s