The way I did it was:
Convert a copy of the Hiren boot CD to run off USB.
Copy the BISO files to the USB stick.
Boot the Hiren USB into the Mini Windows XP
Run the BISO file
When it gives the error about not being able to downgrade the BISO. Do NOT cancel the message and instead copy all the temp files to another location.
Edit the INI file to allow advanced features and also allow downgrades, not sure exactly which 2 lines these were as i'm typing this off the top of my head. You just need to change a 0 to a 1 IIRC.
Then re-run the BIOS file from this new location and it should this time downgrade the BIOS without any problems.