OK, suspecting that there might be more than one "Shutdown.exe" file in the Windows OS, I did the following:
I opened a CMD prompt and typed in"
dir /a/s shutdown.exe [Enter]
The report showed me that there are actually four Shutdown.exe files in Windows.
And, they are not even the same size. Two are 23.064 and two are 53.248.
If I were having your problem, I would find every one of those instances of Shutdown.exe and rename them , one at a time, till I find the one that's causing the problem, then just leave it renamed.
But as for what's calling for a shutdown, it almost has to be some program that you've installed on your computer.
Take a look in Task Manager and disable every program that you don't absolutely HAVE to have running.