Cant install "2026-03 Security Update?

Question

Cant install "2026-03 Security Update? When I download it and go to install it, it said there's an error. Tryed it 3 times now.

Answer 1

Welcome David Bolton!

Thank you for reaching out.

From what you described, the 2026‑03 Security Update could not be completed due to error code 0x800f0991. You’ve attempted to install the update multiple times, but the behavior continues.

The error code 0x800f0991 during a Windows Update installation typically indicates that there are missing files or components needed for the update to proceed.

One recommended solution for this issue is to perform an in‑place upgrade (repair upgrade), which reinstalls Windows without removing your files or apps.

To perform an in‑place upgrade, please see this guide: [How to perform a repair upgrade using the Windows 11 or Windows 10 ISO file]

You can download the official Windows 11 ISO directly from Microsoft here: https://www.microsoft.com/en-us/software-download/windows11

Hope this helps. Please let me know how it goes or if you need further clarification!

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Answer 2

Hi, I'm John! I will help you with this.

To help narrow down the issue:

1. May I know what other troubleshooting steps have you already taken? This will help me avoid recommending them again.
2. Can I ask if you’ve already checked whether you have any third-party antivirus software running?

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If you haven't tried anything yet please start by:

First, make sure there's no third-party antivirus software active that might be interfering with the update process:

• Open the Windows Security app.
• Click the "Settings" (gear icon) in the lower left.
• Under Security Providers, click "Manage Providers".
• Verify if Microsoft Defender Antivirus is the active antivirus and Windows Firewall is the firewall.
• If not, please uninstall the third-party antivirus, restart your PC, and check again at Settings > Windows Update.

If issue persists, run again the Windows Update Troubleshooter from Get Help app:

• Open Settings > System > Troubleshoot > Other troubleshooters.
• Find Windows Update and click Run. This will open the Get Help app.
• Select "I can't download, install, or uninstall an update" and click "Yes".
• This will automatically identify and address any errors and restart the update process. This step is purely automatic.
• Please continue to follow the prompt if the issue persists.

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If the issue persists, please provide more details by answering my questions.

Click "Add comment" below to let me know.

Regards,

John J.D.

Answer 3

Keep gettting "Istall Error - 0x800f0991"

Answer 4

Follow these steps to resolve repeated Windows Update installation failures on Windows 11:

1. Run Windows Update again

• Go to Settings > Windows Update.
• Select Check for updates and try installing the security update once more.
• If it still fails with an error and rolls back, continue with the next steps.

2. Check if the issue matches a known problem

• If the failing update is similar to KB5034765 (Windows 11 security update) and the download reaches about 96% before rolling back with a message like:
  • “Something didn’t go as planned. No need to worry – undoing changes. Please keep your computer on”
  • and Event Viewer shows error 0x800F0922, then install the later cumulative update that fixes it (for that case, KB5035853 is the resolving update).
• In practice, install the latest cumulative update offered in Settings > Windows Update instead of repeatedly retrying the same one.

3. If you see error 0x80070003

• This error indicates some update files (often driver-related) are missing or inaccessible and the system rolls back after restart.
• The most reliable fix is an in-place upgrade repair of Windows:
  • Back up important data first.
  • For PCs or VMs in Azure, back up the OS disk and ensure console access as documented.
  • Perform an in-place upgrade following the guidance in the Windows in-place upgrade articles referenced in the documentation. This keeps files, apps, and settings while repairing system files and update components.
• If error 0x80070003 occurs again during the in-place upgrade, follow the dedicated troubleshooting for that specific error during “Process Drivers for Migration.”

4. Identify the missing path or file (advanced)

• Open %Windir%\Logs\CBS.
• Open the latest CBS.log in a text editor.
• Search for , error and check entries with the same timestamp.
• Note any path or file mentioned as missing or inaccessible; this is what blocks the update. This information is useful if support is needed.

5. If the problem persists

• After trying the above and still failing repeatedly, contact Microsoft Support and provide:
  • The exact update KB number.
  • Any error code shown (for example, 0x80070003 or 0x800F0922).
  • Any missing path or file identified from CBS.log.

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References:

• Error 0x80070003 at startup after you install a Windows update
• February 13, 2024—KB5034765 (OS Builds 22621.3155 and 22631.3155)