Microsoft Defender Antivirus is included for free with Windows 11. As long as you have Windows 11 installed on your device, you can use Microsoft Defender without needing to purchase a subscription. It provides real-time protection against malware, viruses, and other security threats automatically, and it will activate as long as no other antivirus software is installed. If you install another antivirus application, Microsoft Defender will turn off automatically, but it will reactivate if you uninstall the other application.
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