I have seen multiple cases where a key was used "too many times" back closer to release, and the key was blacklisted with one of the service packs when integrated. I've always been under the impression that the new service pack came with an updated blacklisted key database built right in. Unfortunately for you, this seems to be what may have happened.
"What is an SLP product key?
A System Locked Pre-installation key, or SLP key, is a product key that does NOT require activation, and is used by major OEMs (Dell, HP, Lenovo, etc.) with Windows Pre-installation deployments, a special way to install Windows for cloning to PCs en masse.
How can I install an SLP key?
You cannot! SLP keys will NOT install on a standard installation of Windows using standard media (OEM, Retail, Upgrade, Technet/MSDN, etc. etc.) Additionally they may be locked to a particular OEM (ex. Dell.)
The only way you will get a copy of Windows installed using an SLP key is with the original installation media from the OEM, which is usually an imaged based “installation” with all the bloatware on it – maybe except in the case of Dell, which typically will ship Dell branded standard Windows installation media with the SLP key embedded. Point being, you’ll never need to know what an SLP key is, because you’ll never have to enter one during an installation – if you were to be prompted for a key during install, then the SLP key isn’t embedded, and guess what, it won’t work anyway."
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Best Answer
A somewhat weird solution, but it works. Windows 7 has a tool to download the ISO file, but only for retail versions. That means if you enter an OEM key, it will say that your key is invalid because it is OEM. If you enter the key and it is successfull, that means it is retail.
https://www.microsoft.com/en-us/software-download/windows7