From The ESOC wiki for Age of Empires 3
Error 101 occurs if an account has already been created on the CD key associated with your installation. All ESO accounts are associated with the base game, and not the expansions; ensure you are attempting to create an account in the base game.
Sometimes a key can remain from a previous installation of the game. To strip old CD keys from your registry, and force the game to prompt for a new CD key, use the registry scripts here: http://agecommunity.com/downloads/AOE3_Win7_Registry_101.zip
See the official page for downloads and more details: http://agecommunity.com/101.aspx