PRODUCT KEY

Normal Windows Update does not ask for a product key, so stop at that prompt and first determine whether it comes from Windows Setup, an edition upgrade, Activation settings, or an unofficial program.

1. Check activation in Windows

1. Open Start → Settings → Update & Security → Activation.
2. Read the activation status and edition, such as Windows 10 Home or Windows 10 Pro.

Then use the matching case:

• “Windows is activated with a digital license”: no product key is needed. Return to Settings → Update & Security → Windows Update and install updates there.
• “Windows is activated with a digital license linked to your Microsoft account”: it is also properly activated.
• Windows is not activated: select Troubleshoot, if shown. The troubleshooter is available on unactivated Windows 10 version 1607 or later.^1^
• Change product key/Go to Store appears: continue with the licensing checks below.

Do not enter payment information or a key into a browser pop-up or unknown updater. Use only Windows Settings, Microsoft Store, or official Microsoft installation media.

2. Check whether the PC already has a licence

Look for a Certificate of Authenticity (COA) label on the back, bottom, or case of the PC. A valid key has 25 characters in this format:

XXXXX-XXXXX-XXXXX-XXXXX-XXXXX

A preinstalled key may instead be embedded in the PC firmware and applied automatically when the matching Windows edition is installed and connected to the internet.^2^

If you find a COA key:

1. Open Settings → Update & Security → Activation.
2. Select Change product key.
3. Enter the 25-character key.

The key must match the installed edition—for example, a Home key cannot activate Pro. Edition mismatches can produce errors such as 0xC004F210 or 0xC004F034.^3^

3. If a setup or reinstall screen asks for the key

If this PC previously had an authentic digital licence:

1. Select I don’t have a product key.
2. Install the same edition previously licensed, usually Home or Pro.
3. Connect to the internet after installation.

Windows should activate automatically if its digital licence or firmware key remains valid. Installing the wrong edition prevents that automatic activation.^4^

If the prompt offers no I don’t have a product key option, note the exact screen or application displaying it; it may not be the standard Windows installer.

4. If there is no valid licence

• If the computer was sold as a refurbished PC with Windows 10, the refurbisher—not Microsoft—must provide the product key.^4^
• If it was a private used sale and no transferable licence, firmware key, or digital licence exists, purchase a licence through Settings → Update & Security → Activation → Go to Store.
• If Activation shows that a digital licence should exist but Troubleshoot cannot recover it, contact Microsoft customer support and provide the exact activation error code.^1^

Windows 10 support ended on October 14, 2025, so it no longer receives free security fixes through ordinary Windows Update. After resolving activation, check whether the PC supports Windows 11 rather than relying on Windows 10 for continued security updates.^5^

References

1. [Windows 10](https://support.microsoft.com/windows/activation/reactivating-windows-after-a-hardware-change#if-you-have-a-digital-license,-run-the-activation-troubleshooter)
2. Find your Windows product key | Microsoft Support
3. [Windows 10](https://support.microsoft.com/windows/activation/get-help-with-windows-activation-errors#most-common-windows-activation-errors)
4. [Windows 10](https://support.microsoft.com/windows/activation/activate-windows#in-windows-10,-what-windows-activation-method-do-i-use,-a-digital-license-or-a-product-key)
5. [Windows 10](https://support.microsoft.com/windows/deployment/install-upgrade/create-installation-media-for-windows)