Just in case this will be relevant to you I will post:
During my recent Windows 8 woes I had to revert the system to factory default. When I bought the PC I bought Office 2010 in the form of a credit card thing with a pin number on it. The pin was to be scratched, entered into the MS Office website and a Product key was issued. This had all been done for me however prior to collecting the PC and the product key was written on the packaging.
When I tried to reinstall Office using the product key it would not let me - saying it was invalid etc. I tried the backup copy link..to no avail. I had to resort to Microsoft live chat where I gave my product key details. They confirmed it was valid and then posted me a direct download link which then worked.
The hardest part of it was actually finding the Microsoft live chat link.