0xC004C003
Activation server says the key is blocked
What it means
The product key was rejected, often because it was already used too often or is not a real retail key.
Common causes
- A grey-market or reused key
- Wrong edition key
How to fix it
- Check the key matches your edition (Home or Pro).
- If you upgraded for free from an older Windows, use the activation troubleshooter.
- Buy a key from Microsoft or a real retailer if the key was cheap.