while it may have the option to turn it off microsoft may have it set so it shows it runed off but is still running. but in win 10 i hear you cant turn it off. turning off win update should help if thats the case.
i find mostly when mine does it randomly but not frequently is the trusted installer process which is win update. That being said something is writing or reading to your hard drive constantly? checking task manager should yeild a culprit and forcing it closed should help. You can try using and external virus scanner (or portable scanner) to check for viruses as it checks for them by booting with the usb and mounting the os but not running said os to check for viruses.
