OK, I tried everything. My motherboard doesn't boot from USB.
I tried with many mem stick created with many methods, from the very simple "format and copy system files" (yes it can be done using a small utility that enables ghosted check box under windows xp) to the BartPE USB creation and Linux USB distributions. They all boot in the other 3 computers I have, none of them boot with this motherboard.
The memstick is recognized, it apperars in the hard disks sub menu of the F12 booting menu, but it can't boot.