My software cannot be activated because the product key you installed with is not valid. Please uninstall the software and reinstall it using a valid product key.

SOURCE: I tried to use one

Click Start
Click Control Panel
Click Switch to Classic View
Click Add/Remove Programs
Click to select Microsoft Office Professional
Click Uninstall

When the uninstaller completes, restart your computer.

Take the original Microsoft product CD-ROM, and insert it into your CD-drive. It should "auto-run" to start the "installer" program.
Take the product-key that was supplied with the CD-ROM, and use that key to activate the software.

Be sure to be careful:
* ZERO versus OH
* ONE versus ELL
* FIVE versus ESS
* EIGHT versus BEE
when typing the product-key.