how do i turn on windows defender windows 8.1

Windows Defender is the built-in Antivirus in Windows 8.1.

It should be automatically protecting your computer; unless you have another anti-virus installed or you still have an expired trial anti-virus installed, or you are infected with malware.

You have to completely remove any other anti-virus program to start Windows Defender and that includes any trial anti-virus.

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"Uninstallers (removal tools) for common Windows antivirus software"

http://support.eset.com/kb146/?locale=en_US

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How to restart Defender:

https://www.winhelp.us/configure-windows-defender-in-windows-8.html

If you've previously uninstalled a third-party antivirus product (avast!, AVG, Norton/Symantec, McAfee or some other product), you might see a dialog that states that Windows Defender is turned off. In that case, either click Action Center's white flag icon in Taskbar Notification Area (aka System Tray) and click Turn on virus protection (Important) or Turn on spyware protection (Important), or open Control Panel (Windows Key+X has a nice shortcut), type action into Search field, click Action Center and then click one of the Turn on now buttons in Security section.
Please note that Action Center might have no red warning icon for several days after you've removed a third-party virus protection tool.

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A third-party AV/security utility turns Defender off to prevent conflicts