I bought and downloaded this game from matrixgames.com. The installation goes fine but upon startup I get Error 18, which says there is a problem with the serial # as entered. I am a big fan of this series and any help in this matter would be greatly appreciated.

I bought and downloaded the game from matrixgames.com . The installation went fine but on start up I get an error 18, saying there is a problem with my serial #. Im a big fan of these games and any help to fix this issue would be greatly appreciated.