TIMER SHUTDOWN

shutdown -s -t 3600

this will create a countdown timer which will go off in 3600 seconds (1 Hour). you can replace the 3600 to any time limit you want the comp to shutdown ,but remember the time should always be expressed in seconds i.e if u want the comp to shutdown in Half an hour you have to write 1800 and so on...

If you are as lazy as me and don't want to input the command each time, then create a batch file or shortcut

Batch:
open a notepad and type

shutdown -s -t 3600

now in the files menu select "save as" and save this file as Activate.bat

Just double clicking this file will activate your timer

Shortcut:
On the desktop right click and create a new shortcut. in the location type

shutdown -s -t 3600