Windows 8.1 is the first version of Windows that comes with an anti-virus/anti-malware solution out of the box. It’s called Windows Defender that works just like Microsoft’s famous free anti-virus product Security Essentials with frequent daily virus definition data update through Windows Update service.
Normally because it works well and captures the bad files fairly effective people who use Windows 8.1 don’t need to invest in another anti-virus software. However, if you would like to switch to a different one for whatever reason, you may want to disable Windows Defender as well. Running 2 anti-virus software on the same computer usually isn’t a good idea.
But the question is how. Windows Defender is a built-in Windows component that cannot be removed by turning Windows Features on or off. It runs as a Windows service that cannot be stopped or disabled.
You will need to disable it through Group Policy. Open Local Group Policy Editor by typing gpedit.msc in Run dialog box.
Navigate to Computer Configuration → Administrative Templates → Windows Components → Windows Defender. Then enable Turn off Windows Defender on the right side panel.
The change takes effect immediately. If you open Windows Defender screen, you will see red warning sign all over the place warning you that your PC is at risk.
To re-enable Windows Defender again after disabling it, disable “Turn off Windows Defender” group policy and restart Windows Defender service from Services.
Also note that you can do the same to disable Windows Defender in Windows 10 as well.