How to get rid of Windows Defender (Windows 8).

It really problem and direct prohibition does not help here. You can to disable it, turn off and also disable this service at all, but every time You start Windows You'll get the notification, that Windows Defender is turned off and can't protect Your system.
If you want to install another anti-virus program, I recommend you do so:
Open your Windows Defender, then open the "Settings" and uncheck box before "Turn on Real-Time Protection ..." and then "Save changes" . Then go to the "Administrator" and remove the label before "Turn on Windows Defender" and again the "Save changes".
 
Now close Windows Defender completely. Suggested: open the "Task Manager" =>
MSASCui.exe => right click on it => "End Process Tree" , on a pop-up window click "End Process Tree"

Now start Windows in safe mode,
To do this, open the "Configure System" (C:\Windows\System32\msconfig.exe) and select "Boot" => "Safe Boot" => Minimal => 'Apply' => 'OK' => Select Restart now . (back : just the opposite of the same act - you choose the "General" => "Normal Startup" => "Apply" => "OK" and restart ...)
NB! (If your Windows is located elsewhere than on disk C:\, then replace this letter).
Now go to the "C:\Program Files" folder and rename the folder "Windows Defender", such as "Old Windows Defender" or whatever you like.

You need to have administrator rights, and you may need to disable the Windows protection, if You can't rename this folder.

Usually, however, it is not necessary to prohibit any more, because it has already been banned.
If it is needed, then after the PC has rebooted in Safe mode open a new Command Prompt as Administrator and type net stop sppsvc to stop the Software Protection Platform service (sometimes you can get a message that it's already stopped and not running)
(Winkey + R => type there "cmd" or "cmd.exe" (without the quotes) and "OK" )

Note:

Over some time, Windows may solve this problem itself, and you may have no more problem with Windows 8 Defender and with another anti-virus program, what is used/added by user (by You), but it is the case today. (And it is not just mine problem, this is a common problem. Many asking today, how to use some other antivirus without problems and conflicts with Windows Defender in W8. These are lucky people, who can't discover these problems, it does not mean, that these don't exist.)