Not too sure which machine your hard drive is currently in. but if your intention is still to put in the new machine with xp installed, from my recent experiance, dont expect too much to be simple.
When i transfered a disk to a new machine it wouldnt boot at all, in fact i had to run xp repair from the cd which effectively re installed, calling for the key once again. once installed it also asked to be reactiviated. all the programs seemed to be ok and settings remained ok such as e-mail and internet. ms office also had to be reactiviated.
And then the hunt started to track down and install the motherboard bits and vga drivers.
so what seemed a simple idea actually took some time and some frustration to accomplish.
With hindsight a clean install on a new hard drive in the new machine would have been sweeter, and then install programs as required, and then use the transfer wizard to transfer settings for internet and e-mail etc.
So good luck with swapping the disk
Rickity