Windows activation error 0xC004C008 occurs when your activation key is already in use on another device—typical if it's a retail key used elsewhere or still tied to a virtual machine. Here's a clean and effective fix:
- Check your license type – Retail keys activate one PC at a time. OEM licenses are tied to original hardware.
- Remove the key from other devices – If the key’s used elsewhere, log into that device and deactivate or uninstall it.
- Reactivate on your PC – Go to Settings → Update & Security → Activation, click Change product key, and enter your valid key.
- Use phone-activation if needed – If online activation fails, choose the Activate by phone option and follow prompts to connect with Microsoft support.
For a complete, step-by-step guide including advanced activation methods and troubleshooting, visit this detailed walkthrough: How to Fix Windows Activation Error Code 0xC004C008.