Maybe you have a trial version of Office 2007, and the trial period has ended. Or maybe you have not activated your Office software.
What happens if you start Word 2007 and click on Office Button (that's the big round thing at the upper left corner of the Word window) | Word Options | Resources | Activate? If you are already activated, a dialog box saying so will appear. If you are not activated, the Activation Wizard will start. If you have an expired trial version, a dialog box will prompt you to enter a valid product key or convert (purchase a key) online.