Security center is updating definitions for windows defender
Even if you already uninstalled your previous antivirus and you’ve been using Windows Defender for quite some time without any problems, the remnants of your previous antivirus may now be causing the 0x80070643 error.
Completely uninstall your old antivirus programs, restart your computer and Windows Defender should be able to install the latest updates.
In order to get updated antimalware definitions, you must have the Windows Update service running.
Actually, this error has been randomly plaguing Windows Defender users for more than a year now, but Microsoft hasn’t been able to offer a solid explanation as to why this error appears, let alone a fix.
Multiple AV products can cause problems when installed and actively running on the same machine.
See the question "Should I run Microsoft security software at the same time as other security products?
If the interface is not installed, you can add it in the Add Roles and Features Wizard at the Features step, under Windows Defender Features by selecting the GUI for Windows Defender option.
See the Install or uninstall roles, role services, or features topic for information on using the wizard.