You should not disable the default antivirus,Because its now a days windows defender is performing well
How ever if you want to install you can
Windows 8 comes with an built-in antivirus called the Windows Defender. Windows Defender is nothing but the free antivirus of Microsoft called the Microsoft Security Essentials clubbed with the Defender of Windows 7
Disabling the Defender using Windows Services is pretty simple.
1: Launch the Start Menu and search for Services under the Settings tab and click on View local services to launch Windows Services.
2: Look for the service name Windows Defender Service and stop it by right-clicking it
3: Now double-click on the same service to open the Property window and select disable in the Startup Type dropdown list and save the settings.
Your Windows 8 defender will now be disabled permanently, and you can go ahead and install a different antivirus AVAST now
MicrosoftMVP.Support Engineer@IBM-Directory Service & Exchange(,MCSE. MCP .MCSA, CCNA ,RHCE)
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