Windows – Removing “Windows is not genuine” warning

I get the "Your Windows version might not be genuine" warning, even though I have a legit copy of Windows 7. But I can't enter the product key for some reason (it says "Activation status not available" and "Product ID not available"). Since I don't care about the updates (they destroy more than they repair anyway), I just want to remove the warning after boot and on the desktop. But the tutorials I found (like this, archive) say I should

• uninstall KB905474 (which is not installed on my computer)
• delete the registry key HKLM/Software/Microsoft/Windows NT/CurrentVersion/Uninstall/WGA (but the key "Uninstall" doesn't even exist)
• delete C:\Windows\System32\WgaLogon.dll and System32\WgaTray.exe (which both don't exist)
• de-register "LegitCheckControl.dll" (which I also don't have)

Some other tutorials also told me to enter the command SLMGR -REARM, but that gives me the warning "Error 0x80070005", Access denied, action needs higher permissions.

I think you see the pattern: None of the steps of this tutorial work for me and all other tutorials I found just tell me the same as that one. So what can I do? I want an unbothered start and my own desktop background.

It would be great if I could just re-enter the product key, but answers on how to just remove the warning are ok, too.