Welcome Chris van Zantvoort,
Thank you for reaching out to Microsoft Q&A, and I'm sorry for the trouble with activation after your CPU upgrade. Windows can show error 0xC004C008 when the key is recorded as already used on another device, or on more devices than the license terms allow. To narrow this down, could you please tell us:
- Where did you buy the Windows 11 Pro key (Microsoft Store, a retailer, or other)? Was Windows first installed as Home and later upgraded to Pro?
- In Settings > System > Activation, which edition and which activation state is shown? For example, Windows 11 Pro and Not active, or Windows is activated with a digital license linked to your Microsoft account.
- Which Microsoft account do you sign in to Windows with? Is it a personal account, and is it the same one that shows the PC twice?
- Did you also update the BIOS, reset the TPM, or change any other hardware when you installed the new CPU?
In the meantime, please try these steps:
- Confirm the edition. Select Start > Settings > System > Activation and check that the installed edition is Windows 11 Pro, which must match your key. If it shows Home, run the troubleshooter in step 3 and choose the upgrade option if it is offered.
- Check your Microsoft account. Sign in to your Microsoft account, open Devices, and note both entries for this PC (name and last activity). Do not delete anything yet.
- Restart the PC, then sign in as an administrator. Go to Settings > System > Activation > Troubleshoot, select I changed hardware on this device recently, and sign in with the same Microsoft account. Tick This is the device I'm using right now, then select Activate. If two entries appear, try each one. If your device is not listed, check that you signed in with the account that was linked to the license.
- Try phone activation. Type SLUI 04 in the taskbar search, press Enter, choose your country or region, and follow the prompts.
Looking forward to your update.