Gladly stolen from Ramesh
http://www.mvps.org/sramesh2k/
Use Task Scheduler to schedule the computer to shut down and restart at a
specific time.
To do so, follow these steps:
01. Click Start, and then click Control Panel.
02. Click Performance and Maintenance, and then click Scheduled Tasks.
03. Double-click Add Scheduled Task. The Scheduled Task Wizard starts.
04. Click Next.
05. Under Click the program you want Windows to run, click Browse.
06. In the Select Program to Schedule dialog box, locate the
%SystemRoot%\System32 folder, locate and click the Shutdown.exe file, and
then click Open.
07. Under Perform this task, specify a name for the task and how frequently
you want this task to run, and then click Next.
08. Under Select the time and day you want this task to start, specify a
start time and date for the task, and then click Next.
09. Type the user name and password to run this task under, and then click
Next.
10. Click to select the Open advanced properties for this task when I click
Finish check box, and then click Finish.
11. Click the Task tab.
12. In the Run box, specify any additional parameters that you want to use
with Shutdown.exe.
13. Click OK.
NOTE: For exact parameters for shutdown.exe, open cmd and type
shutdown.exe /?
