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Windows shows 0xC004C003 when Microsoft's activation server reports the key as blocked. That sounds final, but two ordinary causes come first, and if neither fits we replace the key.
1. The key is already activated on another machine
A license is used on one PC at a time. If the key is still registered to a computer you no longer use, deactivate it there first, then activate here. If the old machine is gone, write to us with your order number and we will look at your case.
2. The key is for a different edition
A Windows 11 Home key will not activate Windows 11 Pro. A Pro key on a Home PC works differently: Windows switches the edition to Pro first, with no reinstall and no lost files, and the guide chapter Upgrade from Windows 11 Home to Pro has the steps. Check Settings > System > About to see which edition is installed, and compare it against your order.
3. Microsoft has blocked the key
If the key is not registered to another PC and the edition matches, send us your order number and the exact code. We replace a key that Microsoft's server blocks, under the lifetime activation guarantee.
