Hi Alejandro,
When you install a third party antivirus software, it will become your primary or default protection program. If you want Windows Defender to become your default protection program, you need to uninstall or remove the third party antivirus software. You can either uninstall the program from Settings or you can use the uninstaller program of the antivirus software to remove it. We recommend uninstalling the software from the Settings page first. If you're unable to remove the antivirus from Settings, kindly access the developer's website of your antivirus program and look for its uninstaller program. Use the uninstaller program of the antivirus software to remove it from your computer.
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