Take Offline

USB drivers are standard, post Win98.
If you are building your portable WinXP from your existing in-situ installation, it is still possible that it is writing mobo info to the HDD drivers - not necessarily to boot from, of course, but to use to run a HDD once WinXP has booted off the UFD.
However, I assume you've already checked this out?

Anyone have any ideas as to what else it could be?