If you hate to install another application/software just for the purpose to shut down your PC at a scheduled time, you can use a quick and easy command line function to do so. And it is free!

1) Go to Command Prompt from either
a) Start->All Programs->Accessories->Command Prompt
b) Start->Run where a new "Run" window will pop-up. Type in the following and press OK:

2) Another window with black background will appear. Type the following and press enter to schedule your shutdown time (where xx:xx is the time in 24-hour notation):
at xx:xx shutdown -s

For example if I want my PC to shutdown at 6am:
at 06:00 shutdown -s

Another example if I want to shutdown at 10pm
at 22:00 shutdown -s

3) If somehow you want to cancel the scheduled shutdown. Just type the following and press enter (where "a" means abort):
shutdown -a