|Microsoft has a policy of One copy of WinXP/w key for One computer.|
That being the policy MSoft does everything they can to disallow circumvention of this policy.
That means you can't install WinXP to an External USB drive and also that each installation needs to be registered with them within a specified time frame.
So, even if you installed a cloned internal drive in a different computer you would most likely need to perform a repair installation, which would probably require entering an installation key and/or registering.
To sum it up you are not going to run WinXP on two computers using the same key for very long before you are notified you need to register one or the other. The second instance will be rejected.
In addition to ALL the above, copying Windows files to another drive will not get that drive to boot in the new system. Copying will not copy all the files. There are hidden system files that are needed. Imaging the drive/ partition MIGHT get the computer to boot. If the hardware is very similar.