1. Make a text file, shutdown.txt
2. Edit it so it says “shutdown.exe -s -t 20 -f” without the quotes.
3. Save it, and change the .txt to .bat
4. In Vista Task Scheduler, schedule a task at whatever time, and have it run shutdown.bat*

*Schedule a task:
1. Start -> Control Panel -> System and Maintenance -> Administrative Tools -> Schedule Tasks
2. In the menu bar, Action -> Create Basic Task
2a. Fill in name & description, click next to go to the Task Trigger screen.
2b. Tick daily, click next. Fill in the Start Date, and Start Time. In “Recur Every: ____ days” fill in the blank with 1.
2c. You should be a the Action Screen, tick start a program. You will be asked to enter the file path to the shutdown.bat file, IE: C:UsersSleepDesktop.
2d. If you want it to shutdown silently you put “/s” without quotes in “Add arguments”, if you want it to give a pop-up leave the /s out.

If you want an extended detail of the shutdown.exe, type “shutdown.exe /?” into the command prompt.