Re: HDD swap.
If it is XP, it will recognise the new machine as a different PC. XP uses various bits of hardware info to determine a new PC. Not sure of the details, but I believe this includes Motherboard serial, HDD serial (the same if you swap it over), CPU ID, Network MAC address, etc.
So chances are that when you put the HDD in the new machine Windows XP will ask you to re-activate the product.