You may use your activation code on more than one computer that you personally own (see Carla's post for more information), so it could be helpful in troubleshooting this problem to know if this is isolated to only one computer.

We have seen a small number of computers that are apparently blocking the activation code entry dialog window, and we have not yet been able to determine, apart from the details mentioned above, what might be interfering with the process. We have never been able to recreate this issue on any of our lab computers.