You can get Registry Cleaner Here , I would use it to remove all references on that old drive that point to the components on the old system to avoid having driver/config conflicts if you use it as a dual boot drive. RegCleaner can do some nice things for us 98 users ( not sure about XP, although I'd bet it would work nicely if it is supported ).
At any rate, I'd look at using it as a dual boot drive, similar to using a Linux Knoppix CD. Set your BIOS to boot from that drive first, and then when you aren't using it, just disconnect it from the computer so it isn't recognized as your boot disk ( like taking the floppy out to avoid the old Invalid System Disk error. )
Anyone think that this would work too? I'm looking at it like a dual boot scenario, and since it isn't on the same drive as XP, there shouldn't be the issue of which OS is installed first, right? Should just be a matter of selecting your boot sequence.
Get that program though ( it's free ), Just reset the configuration ( see attatched picture )
edit: Oops, forgot to mention....do the reset, shut down the computer, and then remove the HDD. When you reboot the HDD on the NEW system, it'll start installing all the new drivers ( or ask for them )