Going out on a linb here m8, as i've not tested this method, so have no idea if it will work.
It does under Win2K with HDD swaps, but XP's activation thing screws with the process so i'm not sure.
If you still have the old system handy, you could always put the drive back in there.
Then boot to safe mode, remove all
possible drivers manually from device
manager if apps or installers not present.
Try to work in a logical reverse order i.e don't remove motherboard
and system drivers before cards etc.
Shut down, transfer the drive to the new setup, and boot from Windows on the HDD. It should then go about it's detection.
If the activation key issue is still preventing the new detection, you could always try the above but include this...
If you have a copy of XPAnti-Spy
, then use that to disable the licdll.dll, and regwizc.dll that deal with registration, and checking.
You should then shut down, and transfer the drive, and boot as described above in
the new box.
Hope that kinda makes sence, i'm really quite tired at the mo'