If were talking just the base game, then updates would need to go through Impulse/Gamestop (http://www.impulsedriven.com/downloads/impulse_setup.exe). Have you ever registered your serial number with any kind of account, either through Impulse or Stardock?
I honestly can't remember if I ever registered it. I've had it so long that I can't even remember exactly when or where I got it. That's not as bad as it sounds though, I have a box of PC games that I've collected over the years. I counted them one time and there were over sixty, as I recall. It's just hard to keep up with where all of them came from, but I bought them all new, I know that. There's not really a market for used PC games, which is why I still have almost every PC game that I've ever bought.
And that link you provided is working nicely. Thank you very much. ---- Well nevermind. It's telling me that my product code is invalid. I've tripple checked and checked another half dozen times to make sure that I input it correctly. I don't know why it would be saying invalid, unless I did register it before. I honestly can't remember.