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Windows 11 includes Windows Security, the built-in app for Microsoft Defender Antivirus, Windows Firewall, SmartScreen, ransomware-related controls, and hardware-backed protections. You normally do not install it separately.
To activate the main antivirus protection, turn on Real-time protection. Then verify that Defender is the active antivirus provider and check the other Windows Security components that protect your PC.
Turn on Microsoft Defender Antivirus
Use this procedure on a personal Windows 11 PC:
- Open Start.
- Type Windows Security, then select Windows Security from the search results.
- Select Virus & threat protection.
- Under Virus & threat protection settings, select Manage settings.
- Set Real-time protection to On.
- For the recommended Defender configuration, also set Cloud-delivered protection and Automatic sample submission to On.
Real-time protection checks files and programs as you open or run them. Cloud-delivered protection helps Defender use Microsoft’s online threat intelligence, while automatic sample submission allows suspicious files to be sent to Microsoft for analysis.
Windows Security is the current app name. Older instructions may call it “Windows Defender Security Center”; that is outdated terminology.
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Check which antivirus is active
Defender does not normally run its full real-time antivirus protection alongside a compatible third-party antivirus. If you installed another antivirus program, Windows may have disabled Defender automatically.
- Open Windows Security from Start.
- Select Virus & threat protection.
- Under Who’s protecting me?, select Manage providers.
- Look at the Antivirus section to see which provider is active.
If a third-party antivirus is active, you generally have two choices: keep using that product or uninstall it and allow Microsoft Defender Antivirus to take over. After a compatible antivirus is uninstalled, Defender should normally return to active mode automatically. An expired third-party antivirus can also cause Defender to activate.
What to do if Real-time protection will not turn on
| What you see | Likely cause | What to do |
|---|---|---|
| Another antivirus is listed as active | The third-party product has registered itself with Windows | Use that antivirus or uninstall it, then restart Windows and check Manage providers again. |
| The switch is unavailable or settings are locked | The PC may be managed by an employer or school | Contact the administrator. Security policies can prevent users from changing these settings. |
| Windows Security shows warnings after uninstalling antivirus software | The provider registration or security service has not refreshed | Restart the PC, install pending Windows updates, and check the active provider again. |
On a work- or school-managed device, do not try to bypass an administrator’s policy. The organization may intentionally control antivirus, firewall, and notification settings.
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Understand Tamper protection
Tamper protection helps stop applications from changing important Defender settings. An administrator can still change settings through Windows Security when Tamper protection is enabled, but other applications cannot alter them.
This matters if you are trying to disable Real-time protection for troubleshooting: Tamper protection must be turned off first. Turning Real-time protection off is not a permanent way to disable Defender. Windows turns it back on automatically after a short time. While it is off, newly opened or downloaded files are not checked immediately, although scheduled scans can continue.
For normal use, leave Tamper protection and Real-time protection enabled.
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Update Defender before scanning
Activating Defender is only part of the setup. Update its security intelligence before running a scan:
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- Scroll to Virus & threat protection updates or Protection updates.
- Select Check for updates.
To scan immediately, select Quick scan on the Virus & threat protection page.
Choose the right Microsoft Defender scan
| Scan | Use it when |
|---|---|
| Quick scan | You want a fast check of common locations where malware is likely to start. |
| Full scan | You need a comprehensive scan of the drives on the PC. It can take considerably longer. |
| Microsoft Defender Antivirus (offline scan) | You suspect persistent or difficult-to-remove malware. Windows restarts and runs the scan in Windows Recovery Environment. |
After an offline scan, Windows restarts automatically. Check Windows Security > Virus & threat protection > Protection history for the result.
You can also scan one item without opening the app. In File Explorer, right-click a file or folder, select Show more options if necessary, and choose Scan with Microsoft Defender.
Activate or verify the Windows Firewall
Windows Firewall is another integrated Windows Security component. To see which firewall provider is active:
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- Open Windows Security.
- Select Settings.
- Select Manage providers.
- Review the Firewall category.
The same provider view can show the active Antivirus and Web protection providers. If a security suite supplies its own firewall, Windows may list that product instead of Windows Firewall.
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Check SmartScreen and reputation-based protection
Windows 11’s reputation controls can warn about dangerous websites, downloads, files, and applications. Open:
Windows Security > App & browser control > Reputation-based protection
Review the available Microsoft Defender SmartScreen and potentially unwanted app (PUA) controls. PUA protection has been on by default in current Windows configurations since Microsoft began enabling it by default in August 2021, but it is still worth checking if Windows Security reports a warning or a setting was changed.
Smart App Control: an important limitation
Smart App Control is separate from the normal Defender Antivirus switch. Its settings are at:
Windows Security > App & browser control > Smart App Control settings
It has three states:
- Evaluation
- On
- Off
Microsoft limits Smart App Control activation to a new Windows 11 installation. If it was not available when Windows was first installed, you generally cannot enable it on an existing installation unless you reset or reinstall Windows. Also, once Smart App Control leaves Evaluation mode—or you manually switch it On or Off—you cannot return it to Evaluation mode without resetting or reinstalling Windows.
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Review hardware-backed security
Open Windows Security > Device security to inspect the protections provided by your PC’s hardware and firmware. The page can include:
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- Security processor
- Secure boot
- Data encryption
Select Core isolation details to configure core-isolation features such as Memory integrity. Windows 11’s standard hardware-security assessment considers TPM 2.0, Secure Boot, Data Execution Prevention (DEP), and UEFI memory attributes table (UEFI MAT). Memory integrity is additionally required for enhanced hardware security.
If Windows says that a legitimate driver is incompatible after you enable a security feature, first look for an updated driver through Windows Update or Device Manager. Windows 11 can block vulnerable drivers that have known security weaknesses, malware-associated signing certificates, or behavior that attempts to bypass the Windows security model.
If the Security processor section is missing, the PC may not have TPM hardware, or TPM may be disabled in UEFI firmware. A message saying that the TPM is disabled and requires attention usually means you need to enable it in the PC’s BIOS/UEFI settings. The exact menu name varies by manufacturer.
Recommended activation checklist
| Windows Security area | Recommended check |
|---|---|
| Virus & threat protection | Real-time protection, Cloud-delivered protection, and Automatic sample submission: On |
| Manage providers | Confirm that your intended antivirus and firewall providers are active |
| Protection updates | Select Check for updates |
| App & browser control | Review Reputation-based protection and SmartScreen settings |
| Device security | Check Secure Boot, TPM/Security processor, Core isolation, and Data encryption |
Once these checks are complete, run a Quick scan. If you suspect malware that survives a normal boot, use Microsoft Defender Antivirus (offline scan) and review the result in Protection history.
Sources
- Microsoft: Stay protected with the Windows Security app
- Microsoft: Virus and threat protection in the Windows Security app
- Microsoft: App & browser control in the Windows Security app
- Microsoft: Device security in the Windows Security app
FAQ
Is Windows Security already installed in Windows 11?
Yes. Windows Security is built into Windows 11, so you normally do not download or install a separate app. Open Start, search for Windows Security, and launch it from the results.
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Why is Microsoft Defender Antivirus turned off?
A compatible third-party antivirus is usually the reason. Windows normally turns off Defender’s full real-time antivirus protection when another registered antivirus is active. Check Windows Security > Virus & threat protection > Who’s protecting me? > Manage providers.
Can I permanently turn off Defender Real-time protection?
No. Microsoft says Real-time protection turns on again automatically after a short time. If Tamper protection is enabled, you must turn it off before Real-time protection can be turned off.
What is the difference between Windows Security and Microsoft Defender?
Windows Security is the Windows 11 app and interface. Microsoft Defender Antivirus is the antivirus component managed through that app. Windows Security also includes access to firewall, SmartScreen, device-security, and other protections.
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Why can’t I turn on Smart App Control?
Smart App Control can be enabled only on a new Windows 11 installation. On an existing installation, enabling it normally requires resetting or reinstalling Windows. Once it leaves Evaluation mode, returning to Evaluation also requires a reset or reinstall.
The Bottom Line
To activate Windows 11’s built-in antivirus, open Windows Security > Virus & threat protection > Manage settings and turn Real-time protection on. Also enable Cloud-delivered protection and Automatic sample submission, update protection definitions, and confirm the active providers under Manage providers. If Defender will not activate, check for a third-party antivirus or an administrator policy before changing anything else.
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