In order to play most of the game, you'll be needing a registration code. However, as modern machines (ones which run Windows XP, for example) are incapable of running Ducks with any degree of audio support, it seems a little harsh to continue to charge money for the game. This doesn't mean you can get a refund if you registered your copy of the game 4 years ago. Sorry. But it does mean you can download the game (or find where you installed the game on your machine previously, should that be the case) and unlock all its goodies using the following registration code for not a single penny. Hooray!
To unlock the game, select OPTIONS and then REGISTER DUCKS. Enter the name EVERYONE and then the key 004893 and you should be told that the game has been registered.