you want to boot from one hdd that you'll move from pc to pc?
It could work if both computers where identical and it was a retail copy of xp?
OEM instals look for serial numbers on hardware and each install will have different drivers and configurations for the hardware profile.
So if it's retail XP it won't look for the serial numbers and if the computers are identical then the hardware/driver configuration will be the same and should boot.
That's just my guess.