Cannot install Windows Defender updated definitions on SBS 2011 Standard

My SBS 2011 Standard server cannot install updated definitions for Windows Defender (KB915597). Windows Update fails with Error Code 8050A005 ("unknown error"). This problem only affects the server.

I am running Symantec Endpoint Protection so I don't think I need Windows Defender. But I can't figure out how to uninstall or permanently disable Windows Defender on the server or how to un-deploy this update to the server. Windows Updates are managed by Windows Server Update Services.

I tried manually downloading and installing the standalone update package but I didn't see a download for my operating system.

I appreciate your help.
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JohnConnect With a Mentor Business Consultant (Owner)Commented:
I am running Symantec Endpoint Protection so I don't think I need Windows Defender.

You do not need both.  In Local Policies (gpedit.msc) find Windows Defender and disable real time protection.  You can also turn Defender OFF.  

That should disable the updates.
Cliff GaliherCommented:
Windows Defender has meant different things in different versions of windows. In SBS 2011, it doesn't have an anti virus component so it won't install definition updates. Simply don't approve them for SBS. It shouldn't even be trying to install. It should just detect as not applicable.
ClapadorAuthor Commented:
That was fast!  Thank you.  I can't find how to revoke the deployment once it has been approved. (I'm not sure I even intentionally approved it. Updates seem to be automatically approved.)
ClapadorAuthor Commented:
Cliff was right that SBS should not have been updating Windows Defender, but it was and I could not un-deploy the updates.  John's answer showed me how to turn Defender off. The moment I disabled Windows Defender in Group Policy, SBS stopped trying to install Defender updates and the Failed-to-Install error messages in Windows Update stopped. I want to thank both of them for their help.
JohnBusiness Consultant (Owner)Commented:
Thank you for the update.
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