Back in the day, you would have registered it with a service called "Impulse" with the key you had to get updates. If you did so, you can redownload the game from the Stardock store. I see you are posting on a new account though, so you would have to login from whatever account you had then (might not have been aware it was a Stardock account).
I assume if you still have the key somewhere, you could still register it, but I'm not certain on that. I want to say it was on the instruction manual or something, but my memory is fuzzy on the details.
