Defender not installing

Question

How can I run a a malware scan or a virus scan on online? I don't I cannot install a defender.

Answer 1

There is no supported browser-only malware scan; download and run Microsoft Safety Scanner for an on-demand full scan, then repair Windows Security if Microsoft Defender is missing.

Run Microsoft Safety Scanner

Microsoft Safety Scanner can run alongside other antivirus software because it is an on-demand scanner rather than another real-time antivirus product.^1^

1. Download Microsoft Safety Scanner from Microsoft:
   https://www.microsoft.com/wdsi/products/scanner
2. Choose the version matching your PC, normally 64-bit.
3. Open the downloaded file and approve the administrator prompt.
4. Accept the license terms and select Next.
5. Select Full scan, then start the scan.
6. Remove or quarantine anything detected, and restart the computer.

If the affected computer cannot open security websites, download the scanner on a clean PC and transfer it using a USB drive. Do not plug that USB drive back into the clean PC until it has been scanned.

If Safety Scanner produces an error, open %systemroot%\debug, double-click msert, and find the latest error beginning with 0x. Corrupted or missing scanner signatures require downloading a fresh copy.^2^

Another built-in scan option

On Windows 10 or 11, try the Malicious Software Removal Tool:

1. Press Windows key + R.
2. Enter:

mrt.exe

3. Select Yes if prompted.
4. Choose Full scan and complete the removal steps.

This tool targets specific prevalent threats; Microsoft Safety Scanner or Defender Offline provides more comprehensive detection.^3^

Restore Defender afterward

On Windows 10 and Windows 11, Microsoft Defender Antivirus is built into Windows—it normally is not installed as a separate antivirus application.^1^

After the scan:

• If another antivirus such as McAfee, Norton, or Kaspersky is installed and working, Defender may be disabled. Do not run two real-time antivirus products simultaneously.
• If no other antivirus is installed, open Windows Security → Virus & threat protection → Manage settings, then turn on Real-time protection, Cloud-delivered protection, and Automatic sample submission.
• If Virus & threat protection is entirely missing or Defender’s files/services have been removed, use Settings → System → Recovery → Fix problems using Windows Update → Reinstall now where that option is available. This performs a repair installation while retaining files, applications, and settings.^4^

Back up important files before any repair installation. If Windows offers no option to preserve both personal files and apps, cancel rather than continue.

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References

1. Antivirus and antimalware software: FAQ | Microsoft Support
2. How to troubleshoot an error when you run the Microsoft Safety Scanner | Microsoft Support
3. Remove specific prevalent malware with Windows Malicious Software Removal Tool (KB890830) | Microsoft Support
4. in window secutity ther is no section of virus and threat protection - Microsoft Q&A