Firstly, when you start this Application, all planned shutdowns are canceled. After that, you can type in a timespan after which you want the computer to shut down and hit enter or OK or so. If you leave the Application open, it will also show a countdown, but you don't need to. If you want, you can run this Application with any number (seconds) as argument and it will do what you expect it to. On this page you can get a binary: http://goo.gl/LD2dh

You need .NET Framework 3.5 to run this (You most probably already have it)