This post walks you through the process of automatically shut down your work PC at the end of the day. In Windows XP — open Scheduled Tasks from the Control Panel, then double-click on Add Scheduled Task. Click browse button, and navigate to Windows\system32\shutdown.exe. Name the task and choose Daily option. Choose a time to shut down; you'll set a warning and a fail-safe before we're done. The wizard ask for your username and password; you can fill this in or skip it. Before pressing Finish to save the task, check the box marked Open advanced properties for this task when I click Finish.
Note: Whether you're using XP or Vista, once the shutdown is activated; to kill it, you’ll need to launch Shutdown.exe with the switch -a (for abort). So as a failsafe, create a desktop or Quick Launch icon called something like "Cancel shutdown" with the target C:\Windows\system32\shutdown.exe -a.
More info: Windows Vista