Sometimes licence keys can be used for a limited number of times. This is the cause to the error you're receiving. You've probably upgraded the OS and the licence key wasn't deactivated properly.
It's just a matter of contacting the developers to render the key useless and then issue a new one if that's possible and based on your terms.
I've seen that the software is maintained by Avanquest, therefore you have to ask them what is happening with the serial number: http://www.avanquest.com/USA/group/contact/index.html