The product key is not valid - error when activating Office

I am trying to activate Microsoft Office on my PC, but it doesn't work. I get an error notification:
This product cannot be activated, because the product key is not valid.

Before, I was using the trial version of Microsoft Office until it expired. How can fix this and activate Office by using product key ?

Hi,

You can try to fix the problem by repairing Microsoft Office online:

1. Click on Start and open the Control Panel
2. Select Programs and Features
3. Click with right mouse button on Microsoft Office and choose Change
4. Then, the window below should appear. If possible select Online Repair and click on Repair button:



After the repairing, you have to restart the Microsoft Office. If the repair doesn't work, you can try to uninstall and reinstall it.