How can I set my computer to be turned off atomatically in a special time ?

Answer 1

Try to download and install this http://www.download.com/Shutdown-Machine/3000-2344_4-10138258.html?tag=lst-0-1
It works perfectly well and it's free.
It let you specify the time to shutdown.
It also let you set an amount of time to countdown and then shutdown.