Defender won't open

Neil Hodge 5 Reputation points
2026-09-28T18:31:40.8566667+00:00

I am setting up a new laptop and I'm trying to activate Defender through my M365 account but it won't activate

Microsoft 365 and Office | Microsoft 365 Defender | For home | Windows

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  1. John Jefferson Doyon 88,935 Reputation points Independent Advisor
    2026-09-28T19:19:34+00:00

    Hi, @Neil Hodge

    Please check "Settings" > "Apps" > "Installed apps" for any preinstalled third-party antivirus, such as McAfee or Norton. If you intend to use Defender as your antivirus, uninstall that software and restart your PC.

    If the issue persists, please reinstall the Microsoft Defender app:

    • Open "Start", search for "Microsoft Defender", right-click it and select "Uninstall".
    • Reinstall it from the Microsoft Store.
    • Open the app and sign in with the Microsoft account linked to your Microsoft 365 subscription.

    If the issue still persists, to help narrow down the issue:

    1. Does Defender fail to open, or does it open but fail when you sign in? Please share the exact error message.
    2. Which Microsoft 365 subscription do you have?

    Click "Add comment" to let me know.

    Thanks!

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