You didn’t find the following folder on your computer?
C:\ProgramData\McNeel\Rhinoceros\5.0\License Manager\Licenses.
I’m not in front of my Win8 install today but it should be similar to Win 7 AFAIK… If you do have that folder, the instructions are to delete all the files in it and reopen Rhino - it should then ask you to re-enter your license key…
–Mitch