Computer manufacturers often install a trial version of Microsoft Office. If you are getting a prompt to type in a Product Key when opening a Microsoft Office program, it is likely the trial period is over and you'll need to purchase a license (product key) for Microsoft Office.
The product key affixed to your computer is for Windows Vista (the operating system) and not for Microsoft Office. Please review the following:
How to determine whether you have a trial edition of a 2007 Office suite or program installed: