Got a legitimate OEM key? Entering it directly via “Change product key” will result in a message saying “The product key you entered didn’t work. Check the product key or enter a different one.”
Don’t worry, just follow these steps.
Before you start, you should make a Restore point. You’ll have to enable it first, and you’ll need at least approximately 6Gb. I gave it 10Gb to be safe.
- Enter Airplane mode, disable your WiFi, or unplug ethernet. Choose one. Step 3 will result in another error if you skip this.
- Right-Click “This PC”, select “Properties”, then scroll to the bottom. Click Change product key.
- Enter this generic, Microsoft-provided, product key:
VK7JG-NPHTM-C97JM-9MPGT-3V66T. There are other codes for other Windows versions at WinAero.
- Wait for the upgrade to finish.
- Disable airplane / re-enable your Internet.
- Activate using your OEM key.
Done. Do what you want with the restore point.
For reference and to assure you of this process’ legitimacy, read Andre Da Costa’s post, an MVP and moderator at Microsoft Answers.