I had a similar problem after a Windows reinstall. I had copied my games to the new installation (trying to get around reinstalling everything), but Spellforce now asked for a serial number, and refused to let me play.
However, simply reinstalling the game fixed the issue for me. The reinstall also worked in a matter of seconds. Apparently the installer noticed that the files were already there, and just fixed the registry, so that the game didn't ask for a serial any more. Smart. :)