I had that once too, but it was due to trying to install GC2 on a computer that had previously had a different GC2 game on it. It had to do with the email address and serial numbers in the registry not matching the signature file in the game directory.

An email to [email protected] got it worked out though. That would be my suggestion.

In the meantime, you can look in your registry and make sure that your email address is the same that you used to register...

HKEY_LOCAL_MACHINE\SOFTWARE\Stardock\ComponentManager

And the following keys to make sure the serial numbers match your registration...

HKEY_LOCAL_MACHINE\SOFTWARE\Stardock\ComponentManager\Drengin.net\DarkAvatar

and

HKEY_LOCAL_MACHINE\SOFTWARE\Stardock\ComponentManager\Drengin.net\gc2

If they don't, you can try uninstalling, deleting the keys and re-installing.

[obligatory warning] Be careful when editing your registry and only do it if you are sure you know what you're doing. [/obligatory warning]