I vaguely remember this happening once to me. I think what I ended up doing was deleting/uninstalling both the demo AND the full version from my computer, making sure that there were no files left behind, especially saveGame files (could possibly be a little more involved if Windows System Restore is turned on, not sure though.), re-downloading the game and then was able to put in my activation code. I wish I remembered exactly.

Did you download your demos directly from ldw and purchase your games directly from ldw or another vendor?