This is my 1st post which you should have read 1st-
"I recently upgraded from AV 2011 to 2012. After each idle scan the PC shuts off which is a pain. I've actually come back to my PC after a break and notice a Norton pop up window saying Norton has finished it's idle scan and will shut down the PC in x minutes. I've checked all the settings and can't find anything setting that controls the issue."
1st observation was that the Default Idle Scan did turn off the PC when the monitors were shut off already.
2nd observation was the Default Idle Scan was already running and the pop up came up sometime after the screen saver disengaged.
Both actions are quite quite different than when a computer freezes due to a heavy load on the CPU or GPU or bad program code that goes on and on until Dr Watson shows up ;-)
Cheers