If this were Unix or Linux we were talking about, it'd be absolutely no problem. However, Windows has always had spotty support for this sort of feature at best. It works fine for some people, and not at all for others. You can try setting up a scheduled task to shut the system off, but I wouldn't get your hopes up on it working.
The most reliable way I can think to do this, is to just get a timer for the electrical outlet which cuts the power at a certain time. Over time it might be a little hard on the computer, but by and large it's not much different from the freequent rebootings a lot of people have to do from using Windows anyway.
I would like to have a computer shut off at a certain time of day...irregardless of user.
User has tendency to forget to shut down...