Wouldn't one come with the game? Or did you pirate it off the internet? If you pirated it, I'm sure the same website would have a way around this problem for you.
Google search my friend, with the o.g. Halo I'm sure you can find it somewhere. In fact Google is the reason I have the full version of Feeding Frenzy 2; some lovely person put their registration code up for grabs for everyone on the internet to use.