Do you have it loaded? I'm not sure about 2004, but in 2006, TC-3 and 2009, your S/N is stored in the settings folder in the config.dat. Use notepad to open it.
If this is a new "box" purchase, the serial number will be located either on the back of the little manual provided with the disk or will be on the disk jacket itself.
If you purchased from the Auran store via digital download, the serial number will have been mailed to you with the instruction on how to download the product.
If this is a "used" purchase such as what EB games sells, you will be out of luck as a serial number can be registered only once.