G'Day
If you are asking if you can use the same disk to install XP onto two computers at the same time - No that is illegal.
If you have a retail version and you want to install XP onto the new computer after removing XP from the old computer - as far as I know that is legal - it is no different than having a computer die and installing it onto the replacement, that I have done in the past with no problem.
If you have an OEM (preinstalled) disk, using the disk to install onto another computer is also illegal - the OEM version is tied to the original computer - although how they can enforce this I am not sure.
If you have a Recovery/Restore disk, you wont be able to use it on another computer at all.
Geoff