In case you accidentally format your drive before installing the upgrade disc... No registry edits or anything...
Just had the trouble of accidentally going the Custom > Format route for upgrading my Windows computer. It ended up formatting and erasing my old existing Windows before installing the upgrade. Unfortunately, that meant the upgrade key was not supposed to work anymore.
An easy way to resolve it is to just leave the key blank so that you can use Windows for 30 days.
Once logged in, create a folder "Windows.old" in the C:\ drive.
Then go to Control Panel to activate your Windows copy. The upgrade serial key will work fine.
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