Thank goodness for disc cloning software.
Any blue screens, beside the black one?
Ok, I'm guessing you're already runing your Windows on sp2 again.
Here is something similar, http://en.kioskea.net/forum/affich-10056-windows-xp-reboot-loop-help
from sp2 to sp3 ,the regfix part, but unfortunatelly, I haven't had the chance to test this yet.