The game licence states that it cannot be transferred. So if you DID buy the game second hand, technically uou are not allowed to use the serial number to login and get patches and play MP. If you are lucky Stardock and/or IC may help you out anyway.
As a general warning - don't buy PC games used and expect to be able to use the serials - it is becoming more and more common for this to be the case as products like Impulse allow a person to download the game - hence they can buy the game, register it and return/sell the box and get a free copy...