I also have this problem. Whenever I reboot my computer an error message saying windows defender is not turned on pops up. I click turn on and it does so. But it keeps coming up every time I reboot. I think it all started after an update to windows defender failed. The service is set to automatic. Is there any way to suppress this error message or get windows defender to stay turned on?
Edit: Even though I set the service to automatic it keeps switching back to manual. I tried to to start it from the services panel and it popped up an error message saying that the service started and stopped. Are there other services that Windows defender might depend on that somehow got turned off?
I'm running AVG Free 9.0, but it hasn't conflicted with defender before this. I never use defender so I wouldn't mind figuring out a way to just make this error message stop instead of getting defender to turn on. Not much to report from event viewer other than defender being enabled and disabled several times.
Edit: Solved my own problem, since I don't need it to work I just stopped defender from running on startup.
I am not sure if stopping it at startup, will avoid the popup, but it seems to have worked for you. But if you wish, it is easy enough to turn off the popups. They are mostly being generated by the Action centre. I do not have any use for this facility, and have it also turned off. But if you open the "Notification Area" you can select the Action center and click " Hide Icon and Notification"
Alternatively, you can open the action centre in the Control panel and change the options for notification.
edit later. I didn't address the thread title. Several Antimalware programs are turning off Defender by default. The latest AVG, is one of such. Microsoft security essentials turns it off also, by default. If you are running a third party anti-malware, you do not really need defender and best to totally disable it through "services.msc"