> **Note:** This answer does not necessarily reflect EU laws, but it is valid to US laws to the best of my knowledge. I am not a lawyer, and this is not replacement for actual legal counsel for this issue.
The short answer is, you can't.
If you bought this computer new and it didn't come with an actual product key, the key itself came bundled with the computer and the computer's BIOS. As there's no way to remove this key, you technically can't return anything, and there really isn't a way to invalidate this OEM key, as computers come from the factory pre-activated with the license already…