<t>The safest way to do this is to uninstall Avast! (in Programs and Features) and then to reinstall it. That will ensure that MS hasn't removed any part of the Avast! suite's protection. Reinstalling Avast! will then disable Defender correctly.<br/>
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Merely shutting Windows Defender does not guarantee that third-party AV will be working properly.<br/>
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Postscript: As has noted in comments, recent Windows updates reinstall Defender, even though third-party AV has already been installed. There is not much impact, I find, except on cold boot, but scheduled scans can be disabled in Scheduled Tasks. For example, using Nisoft's TaskSchedulerView, right-click on an item and select Disable.<br/>
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Also turn off scans and the alert message in the Windows Defender Security Center that Defender.<br/>
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