Windows Defender Offlinehelps protect your PC by scanning it to remove rootkits and other advanced malware that can't always be detected by antimalware programs. If this type of malware is detected on your PC, you'll be prompted in Microsoft Security Essentials or Windows Defender to download and run Windows Defender Offline

And we have a good FAQ to help answer your most common questions

 

Also check our Windows Defender Offline system requirements before you start

Learn more about Security, privacy, and user accounts in Windows

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