You know those lame software that you have to pay for so they can do a 'timed shut down your PC'? Why buy when Windows already comes with a good enough method?
However, a lot of people will have problems using the command line itself, as giving parameters in a CLI is not the most user-friendly.
That's where Window's shutdown GUI comes in handy.

Goto Start -> Run and type in "shutdown /i" (without quotes)
This will bring up the GUI. To close your own computer, click 'Add' and put in 127.0.0.1 (localhost) in the text field, and click ok.
You can select the operation (shutdown, restart, logoff), and set how long you want the warning dialog to be there for (in seconds) before the computer shuts down. Type in some text in the Comments section, then click ok and the shutdown dialog will show up.
If, for any reason, you want to cancel the shutdown, go to Start -> Run and type in "shutdown /a", and it will be aborted.