How to disable Windows Defender in Windows 10 permanently

1. Tap on the Windows-key to open the Start Menu.
2. Type regedit.exe and hit enter.
3. Confirm the UAC prompt to continue.
4. Navigate to the following key: HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender
5. If you see the preference DisableAntiSpyware on the right double-click on it and set it to 1 to disable Windows Defender.
6. If the preference does not exist, right-click on Windows Defender and select New > Dword (32-bit) Value, and name it DisableAntiSpyware.
7. set value to 1
8. disable windows defender icon on systray using taskmanager

Source: GHacks.net