After some tests i figured out, that i can not use the install to USB from the
I386 UBUNTU version.
It will not work together with a big HDD (i used 320GB).
It works somehow if i use the full install mode, but...not really.
Also the HP Tool will not work out if you gave it some DOS bootable files because of the large HDD which is not accessible by the old DOS format.
The main problem here is, that most of the old PC's are not able to boot from
a FAT 32 partition (only FAT 16).
Did you try out GRUB (GRLDR windows xp) on an old PC which boots only USB-FDD?
I would like to figure out a compatible setup, maybe the boot loader from Vista can do it somehow (i never tried Vista at all, but maybe it's good for it).