I had an idea - why dont I download a new installer off the website and try and reinstall it - wont mess anything up will it?
No installer ever mess with a registry in the serial and no installer will clean it up.
Only you can have inserted a serial number in the registry and you must have done that manually, because when you use the normal "Register Serial" option in the Addon Manager, the Serial Number you type there won't be saved in the registry, unless it has been activated successfully.
So, either activate it, if it's a proper serial, or remove it, if it's not.