Win8
Enable
bcdedit /set {default} bootmenupolicy legacy
Disable
bcdedit /set {default} bootmenupolicy standard

Win10
Enable
bcdedit /set {bootmgr} displaybootmenu yes
Disable
bcdedit /set {bootmgr} displaybootmenu no


Use the Windows key + R keyboard shortcut to open the Run command, type regedit, and click OK to open the registry.
Browse the following path:

HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender

If you don't see DWORD DisableAntiSpyware, right-click on an empty space, select New, and click on DWORD (32-bit) Value.
Name the key DisableAntiSpyware.
Double-click the newly created key, and set the value from 0 to 1.



    Name the key Real-Time Protection and press Enter.

    Right-click on the Real-Time Protection (folder) key, select New, and click on DWORD (32-bit) Value.
    Name the key DisableBehaviorMonitoring and press Enter.
    Double-click the newly created key and set the value from 0 to 1.

    Click OK.

    Right-click on the Real-Time Protection (folder) key, select New, and click on DWORD (32-bit) Value.
    Name the key DisableOnAccessProtection and press Enter.
    Double-click the newly created key and set the value from 0 to 1.

    Click OK.

    Right-click on the Real-Time Protection (folder) key, select New, and click on DWORD (32-bit) Value.
    Name the key DisableScanOnRealtimeEnable and press Enter.
    Double-click the newly created key and set the value from 0 to 1.

    Click OK.

After completing the steps, simply restart your computer to disable the Windows Defender Antivirus permanently.

If you change your mind, you can always revert the changes using the same instructions, but on step No. 3, right-click the DisableAntiSpyware key, and select Delete. Then inside the Windows Defender (folder) key, right-click the Real-Time Protection (folder) key and select Delete to remove key and its content. Finally, restart your device to complete reverting the changes.