This happens if you reinstall quite a few times (or install on multiple systems).
I do not work for Stardock, but I do know that you can have your account activations reset allowing you to install again.
I suggest sending in an e-mail with the subject line: "Reset activation account". This should flag the message to be sent to the correct tech to reset your account. Just make sure to include your e-mail address and serial number in the message to help them find the correct account to reset.
Note - I have had to do this a few times, especially during the WindowBlinds 5 beta period.
Good luck.