If it's a 2003 PEKTRON system then you need to ensure the fob comes with its original barcode - or you wont be bale to link it to the ECU. You'll need access to a T4 diagnostoc system or a PSCAN to do the linking... fobs can only be reused if the barcode is present.
If it's a LUCAS system (i.e. a 2002 car that was registered in 2003) then no need for the barcode as they can be linked anyway as far as I am aware. All working fobs can be reused.