1) Open the Run window (Windows Key+R) or (Start / All Programs / Accessories / Run).
2) type

shutdown -s -t 1800

[1800(can be changed) is the number of seconds after which the computer will turn off.]

Click on OK and your computer will turn off after the specified time (here 30 minutes).
You can change -s for alternatives as below:
options    effect
-l         to log off
-r         to reboot