When you purchase XP and install it you agree to the EULA which prohibits the use on more than one computer. So you are breaking the copyright law.
But as has already been stated you have to "activate" XP and when you come to "activate" the second computer Microsoft would recognise that is was not the original motherboard, etc. and refuse to "activate" the program.
Whatever you think about it, you have to buy one program for one computer.
This thread is now locked and can not be replied to.