[scrotnig]

From memory I think the serial number is only needed if registering without the CD. All it actually does is tell the reg server whether you have cable modem or a set top box.

The PIN is specific to you, and is needed whichever method you use to register. You'll be asked for PIN and surname, but you only need to do this once. All the PIN does is identify your broadband account, so that when the modem is registered by the system it knows whose account to add it to.