Task Scheduler is a process that runs within the operating system. If the OS
is not running, neither is the process. So, it cannot be used to start the
OS. Some pc BIOS' have settings that will allow you to start it at a
predetermined time, you will have to delve into yours and see if these
options exist.
Under the Conditions tab for the task, place a check in front of "Wake
the computer to run this task." In order for the computer to wake up
it must be placed in sleep mode.