Your reply seems hard to understand...will do my best
Identical systems will not have the same key as most pieces of hardware have a unique ID. CPU's do and so do network cards and hard drives....video cards i think are the same.
MS will be able to tell. you can only run windows on one machine at a time. If you windows is OEM (came with a computer or got it OEM from a computer store, plain case) it is locked for the most part to your hardware. If you have the retail copy (staples, futureshop , best buy, radio shack ect either way it has a bit fancy box) you can activate it on a new machine as long as you take it off the old one(legally at least)
What do all your old computers have for an os? XP
do you want to use your old XP on a new machine? or new XP on the old one?
If you are building a new computer you should be able to get an OEM copy of windows XP MCE(media center edition, its the same as pro with the media capabilities added) for 120-140 or so from the store you get the parts from.
So in short when you active windows it will send MS the hardware info in question....and if you reactivate often or the id is changed you will have to call ms(the activation windows will tell you to call)
Bottom line...you need a copy of windows for each computer(legally). I am not getting into the conversation of software piracy....if its working now...use it...if not call ms if its genuine. they will help you....if its cracked....well you kind of on your own....