[pip22]

You cannot move Windows XP (or Vista) from one PC to a different one. The fact that the two hard drives are identical or even similar is irrelevant.

You have to perform a complete re-install of Windows XP to do that, and even then you would find that some devices won't work because you need XP-compatible drivers, and most of the drivers supplied by HP on their website for your new computer will be for Vista only.

Put the new hard drive back into the new computer. Either leave Vista on it (as HP intended) or totally wipe the disk and install XP on it (but bear in mind you may have problems getting XP drivers for some of the internal components).