Windows 8 comes with pre-installed Windows Defender to protect your PC against malware, spywares on viruses that may cause harm to your computer. But if you are more satisfied with some other anti-virus then you need to disable Windows Defender First.
Windows Defender was initially launched to protect windows from malware but now it has been re-developed into a complete antivirus program that is available for free download from Microsoft. Windows Defender can provide you complete security for your PC but still it is bit heavy while using resources and thus gets slow.
So to install some other antivirus, you need to disable Windows Defender Service first. To disable Windows Defender, go to your start screen and search for “services” in Settings tab and launch View local services.
Now in the list, look for the Windows Defender Service and stop it by right clicking on it.
Doing so will turn off Windows Defender Service for now but it will automatically start again when you will reboot your computer. So to disable it from startup, right click again and got to Properties and from the Startup Type drop down list, choose disabled and save your settings.
This way you can turn off windows defender and now you can install any other antivirus of your choice.
You can disable Windows Defender Service through a more easy way. All you need to do is to launch Windows Defender and goto Settings tab and in under Administrator section you will find an option to Turn Windows Defender Off.
So you can permanently disable Windows Defender and install any other antivirus on your Windows 8 PC and if you want to turn it on again, simply follow the same procedure to turn on windows defender service.