hi..
i haven't used the option of booting from USB drive but BIOS gives u option..so, it must be possible...
u can make a bootable Floppy disk using windows..copy and paste all of the contents in a USB drive..i m not sure but it might be enough to boot from this drive..also set first ur booting priority on external device.
D.O.S. and the BIOS is the answer they both search for a bootable system and if cannot be found goes to basic. to boot from USB that disk must be a bootable system otherwise it will not boot from there.
as they said , it depends whether your bios support that or not,,if it doesnt support you can update your bios online ,,but take care it is a very dangerous operation and must take care also that the PC will not shut down suddenly ,,