0xC004C003
Microsoft's server says the product key is blocked or invalid.
How to fix it
- Check the key was typed correctly.
- If you upgraded from an older licensed Windows, connect to the internet and wait: activation can take a day.
- If you bought the key cheaply online, it may be a resold or revoked key: ask the seller for a refund.
Is it serious?
A key problem, not a PC problem.