Does your user account have a password? IF your account doesn't have a password, Scheduled Tasks won't run.
What error is shown for the scheduled task in the Status column?
Check the log for more info:
In the Scheduled Tasks window click Advanced | View Log
Here are the full instructions
Open Control Panel.
Double click on Scheduled Tasks.
In the Scheduled Tasks folder, double click on Add Scheduled
When the Scheduled Task Wizard starts, hit the Next button.
After a short time you should see a box with a Browse button
Click on the Browse button.
This should bring up "Select Program to Schedule".
Navigate to the C:\Windows\System32 folder.
Scroll through this folder till you locate shutdown.exe.
Click on shutdown.exe so that it's listed in the "File name"
Hit the Open button.
Back in the Scheduled Task Wizard, select an appropriate name
for this task like Shutdown.
Select Daily under "Perform this task".
Hit the Next button.
Enter the time you want the computer to shut down.
Hit the Next button.
Enter a user name and password.
Note: The account you use to run this task must have a
On the next page, put a check mark in the box next to "Open
advanced properties for this task when I click Finish".
Hit the Finish button.
You should now see the properties sheet for the Shutdown task.
Click on the Task tab.
In the Run line, you should see the following:
You need to add -s -t 00 to this entry. It should end up
looking like this. or whatever time you wish.
C:\WINDOWS\system32\shutdown.exe -s -t 00
Next, click on the Settings tab.
Put a check mark in the box next to "Wake the computer to run
this task". That way, if the computer is on standby, it will
still be shut down.
Additionally please bear in mind that you need to ensure that automatic updates etc or any other management program is not running and downloading/installing at this scheduled time, otherwise you will of course have problems.
The task is running and exiting without returning an error, which usually means a syntax error. (The error code is passed to the cmd.exe shell, but it doesn't pass it to the Task Scheduler; the exit code of 0 is from cmd.exe exiting).
If you forgot the space before the -s switch that would do it.
The command has to be shutdown -s
This will display the shutdown dialog with the default 30 second countdown.
Edit the current task command line, or delete it and create a new one using the steps Macboatmaster has posted
The -t lets you set the time of the countdown, with it set to zero as above, it will shutdown immediately with no dialog.
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