I'd say the easiest way is to put the new drive in an external caddy, and use Seagate's (HDD manufacturer) own cloning tool Diskwizard, which is a slimmed down version of Acronis, but free
You should be able to find the COA number in case you need it with magic Jellybean from here
There should be no re authentification issues if you clone/image the existing drive to a larger one
Depending on the new drive manufacturer if you choose one of the better known brands they will most likely have some free drive cloning/imaging software available on their web site for download often this is based on the Acronis products which will make the project easy for you
If the external drives are SATA (which by the sounds of it you aren't sure of), are you certain that the existing lappie drive is SATA as well? I ask as I'm guessing the machine is a few years old (as the OS is XP) and might have an IDE drive and you won't be able to swap out an IDE and insert a SATA (or vice versa for that matter).
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