There are two parts to this which you need to complete; one in Windows Defender and the other in the services menu. This ensures that nothing about Windows Defender works and that even the background tasks are disabled and turned off.

VistaBase does not recommend that you alter these settings as Microsoft Update provides your computer with important updates that make your computer safer and more secure.

1. Go to Start, All Programs, then Windows Defender.
2. Select the Tools option at the top of the screen.
3. Under “Settings”, click the Options button.
4. Untick the “Automatically scan my computer” checkbox, the “Use real-time protection” checkbox, untick the “Use Windows Defender” and the “Allow everyone to use Windows Defender” checkboxes.
5. Click the Save button. If prompted for administrators access, click Allow or enter in the password.
6. Close Windows Defender. If the icon remains in the taskbar, right click it and select Exit, then click Yes.
7. Go to Start then Run (or press the Windows key +R).
8. Type in services.msc then press OK.
9. Scroll all the way down the page, then double click on Windows Defender.
10. Under the startup type, select Disabled.
11. Under service status, make sure the service is stopped.
12. Apply then OK, then close the Services console.

You may need to restart for full effect, but this will stop Windows Defender from scanning anything on your machine and should save you a good chunk of memory.