First if you access the BIOS, then under power management there will be a timer setting allowing you you to set the startup time/s or shutdown time/s of your pc.
NOTE: these settings are dependent of your BIOS version

Secondly you can select power management and specify after what time the pc should turn its self of. This can be used to turn of different devices eg. screen, hard drives or send it into hibernate. This will also be help full 'cause if the pc is awaken out of hibernation it will prompt for your password.

Alternatively you can set a <scheduled task> wich is very specific it asks the day, month and even year that you want it to turn it self of.