Sorry, i should have been clearer.
Yes, exactly
I mean you swap the drive from it's current MB/CPU/RAM/VID hardware to a totally new one.
A lot of the time doing this on XP results in a BSOD or a windows activation error because it expects to find the hardware it was first installed on.
But if you do it during the service pack reboot it does a hardware redetect and doesn't have the problem,
Oh and if you're already on the latest service pack, so can't upgrade to anything, I have had good luck rolling it back to a past service pack using an old restore point and then replying the service pack.
From memory there is another way to force a hardware re-detect, i can't remember what it was but i do remember it being easier to just use the service pack reboot method.