Go to key My Computer>HKEY_LOCAL_MACHINE>SOFTWARE>Microsoft>Microsoft Games>Age of Empires 3>1.0
Click on 1.0: - You will see a bunch of text appear in the right hand window
One of the entries should be called "PID". Delete it.
You will be prompted to enter a CD-Key. Enter the desired CD-Key. (An invalid key will be met with an error message; a valid key will launch the game)