Windows Activation issues error code 0XC004C008

Question

Windows Activation issues error code 0XC004C008

Andrea Angelini 0

Screenshot 2026-11-12 210318

Dear community, i assambled my new freshly PC and bought a retail copy of windows 11 pro at a retail shop and spent 75€ for the license, the retailer told me they bought their license keys from a legitimate microsoft distributor.
I tried the first time to install windows normally and filled in the activation licence key when prompted, it was then recognised as Windows 11 Pro. the installation afterwards proceeded through half way of the process when suddenly the screen goes black and 2 consecutive graphical glitches occured on the screen, by that time i connected the desktop to the phone via usb tethering. i couldn't proceed with the completion of the installation setup and restarted the desktop, yet, after doing so, there was always this black screen and i was unable to continue in any way the installation. i decided then to start a fresh installation again repeating the same process, thus windows was creating other 4 more partitions on my SSD M2 drive, i deleted completely those partitions so i can start the windows installation again, i repeated the installation process and i choosed to activate windows later so i can speed up the setup and installation process so i can finally install the motherboard's drivers and avoid the black screen whereas it's origin was unknown, upon restarting multiple times the desktop, i finally got time to activate windows, went to the system and clicked on change product key, i entered the same 25 digits as i was provided from the retailer only to get this error that the windows product key was used on a different machine.

i never used this key anywhere else, it was the only machine at the same time with a 15 minutes later or reuse on the same motherboard, same SSD, same machine, same Desktop, same account same day!!!!!!!!!!!!!!!!!!!

WHAT SHOULD I POSSIBLY DO NOW?

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2 answers

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Answer 1

Error 0xC004C008 means the product key is already recorded as used on another device, or on more devices than allowed.

For this scenario, use this order:

Open Start > Settings > System > Activation.
Run Troubleshoot.
If the device had a substantial hardware change or activation state mismatch, try reactivation from there.
If activation still fails, run phone activation:
- In taskbar search, type SLUI 04
- Open SLUI 04 and follow the on-screen activation steps
If phone activation does not resolve it, contact the retailer that sold the key and ask for a replacement key or validation, because Microsoft states this error can occur when the key has already been used.

Also verify these common causes:

The installed edition must match the key. A Windows 11 Pro key must be used with Windows 11 Pro installed.
If the key was entered during an earlier failed setup attempt, the activation service may still treat the key as already used.
If the PC was recently repaired or rebuilt, reactivation may require the troubleshooter or phone activation.

If the key came from a reseller and the device was expected to activate on first use, Microsoft guidance is to contact the reseller. If the key remains blocked after troubleshooting and phone activation, a different valid key may be required.

References:

Answer 2

I think I know what happened! You put in your key during the initial installation, and it crashed while you were online through the USB tether. This means that the key was activated at this point, before any chipset/network drivers had been installed. This will mean Microsoft's activation servers will have recorded some slightly different hardware information. It will appear to them as if it is a completely separate PC.

To get around this, you can activate using your phone. Open slui 4, select your country, and when it comes to the help screen explain that you have re-installed on the same PC due to a failed setup. Most of the time retail keys will be approved.

If this doesn't work, return to the shop with your receipt. As the key is a retail key and has only ever been used on one PC it should not be blocked. They need to either resolve the issue with their distributor or provide you with a working key.