My pc used to be on windows 8.1 and I never had this problem. Ever since I installed windows 10 I have noticed that it has been randomly starting up. I shut it down and then wake up the next day finding that it had randomly turned on during the night. It shows the log in page as usual with no signs of being locked, standby or anything else. it shows as if i shut it down and turned it back on again manually but i haven't. This only started to happen after I updated windows 8.1 to 10 so i dont think it has anything to do with the motherboard