On October 7, 2015 the Windows Defender icon appeared in the system tray of my Windows 10 system. Windows Defender was fully active even though Norton Security had been installed in January, 2015. On another Windows 10 machine I have Windows Defender being active with Norton Security installed was preventing Windows update from functioning. I implemented the following steps on October 7. Then Windows Defender reasserted itself today, November 2, 2015, and I followed the same steps. In Windows, under “Settings -> Update & Security -> Windows Defender”, then under "Real-time protection" is the statement "You can turn this off temporarily, but if it's off for a while we will turn it back on automatically". Since this just started in early October, with what I assume was a Windows update, it would appear that something has changed in Windows or the Windows philosophy. Here are the steps I implemented:
In the system administrator account:
1. Under “Settings” -> “Update & Security” -> “Windows Defender” change the three indicators to “off”.
2. In the Registry Editor:
1. Go to HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
2. Create a new string value, “Windows Defender” and modify its value to "%ProgramFiles%\Windows Defender\MSASCui.exe" -runkey
1. N.B. This is a string value so that the two instances of the double quotes (") are a part of what is entered.
3. In the Task Manager, Startup tab, disable the Windows Defender User Interface.
4. Sign out/in, restart or shutdown/startup.
This seems to work, at least for a while. I did notice that after completing these steps, in my normal, i.e., non-administrator account, under “Settings -> Update & Security -> Windows Defender”, that "Real-time protection" is shown as "on" and grayed out preventing me from changing it. I did check to see that Windows update, i.e., forcing it to check for updates worked. On the machine in which Windows update was not working, it would cycle for 20 minutes and then give an error code associated with Windows Defender being unhappy.
Comments on related experiences are welcome.