Doing some digging on the web, it seems that various users are having the same problem: They performed an upgrade to Windows 8 Pro or core edition and then suddenly they lost the ability to open Word, Excel, PowerPorint, etc., even after trying re-enter their valid product key, the software will respond with a message telling the user that the product key is invalid. If you are having the same or related problem, you should consider picking up the phone and calling Microsoft Support and doing an activation by phone.

Key in mind the terms for the license:

Microsoft Office Activation Number: Activate Microsoft Office 2007 programs (See activation by phone) Older version of Office: Activation and registration of a Microsoft product Invalid product key type errors occur in Microsoft Office 2007 or 2003 Phone Activation: Activate by using the telephone You can telephone an Activation Center and activate your product with the help of a customer service representative. For more information refer to this answer at Microsoft   All content on this site is provided with no warranties, express or implied. Use any information at your own risk. Always backup of your device and files before making any changes. Privacy policy info.