Windows Defender for Windows 10 and other Editions

Microsoft introduced Windows Defender Advanced Threat Protection to enable customers to detect, investigate and respond to Advanced Attacks on their Networks.

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: Post breach detection for active attacks actionable, correlated, real-time and historical for known and unknown adversaries

: Easily investigate & exploreenterprise endpoints to understand scope of breach through rich machine timeline and data pivoting

: Self hunting across protected assets search current and historical observables – machines, files, URLs or IPs across all protected endpoints. Deep file analysis for files retrieved from endpoints or from Microsoft’s global sample store

: Built in threat intelligence knowledge base provides actor and intent context for every threat intelligence-based detection – combining 1st and 3rd party intelligence sources

Windows Defender is built into Windows 10 and you do not need to download it.

Windows defender for Windows 8, 8.1 and Windows RT does not require you to download or install Windows Defender. Windows Defender is already built-in and provides protection against malware.

Source: http://windows.microsoft.com/en-IN/windows/security-essentials-download

Windows 8 onwards, Microsoft have replaced Microsoft Security Essentials, you don’t need to download it separately—Windows Defender is already included and ready to go.

But if you’re looking to protect a PC with an older version of Windows like Windows 7 or Windows Vista, you can use Microsoft Security Essentials to help guard against viruses, spyware, and other malicious software.

Troubleshooting Windows Defender is turned off or not working

Check if you have any 3rd Party Antivirus program which might be causing issues with Windows Defender.
Open Action Center and see if you are able to enable Windows Defender.
Run services.msc to open Services Manager. Ensure that the Windows Defender service is started and set to Automatic.
You can try to re-register the following dll files and see if this helps you. Run each command at the command prompt one by one and hit Enter:

regsvr32 wuaueng.dll
regsvr32 wucltui.dll
regsvr32 softpub.dll
regsvr32 wintrust.dll
regsvr32 initpki.dll
regsvr32 wups.dll
regsvr32 wuweb.dll
regsvr32 atl.dll
regsvr32 mssip32.dll

Try enabling Windows Defender from the Registry.

HKEY_LOCAL_MACHINESOFTWAREPoliciesMicrosoftWindows Defender

DisableAntiSpyware DWORD

(delete) = On
1 = Off

If still it does not work, then please contact Microsoft Support.

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