To activate an OEM Windows system you need three pieces of a puzzle that must fit together: Product Key, BIOS SLIC, and OEM Certificate.
Since Windows 7 has been running on your system, you can be certain that you have the correct BIOS SLIC (System Locked License.)
You also have the correct Product Key.
That leaves the OEM Certificate. If you did NOT install your OEM Certificate, then you can assume it's missing. The easiest fix is to grab a copy of it from your old disk. To do this, you have two choices: Manually or use Software. . .
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