First off... you have the latest BIOS. You should be able to boot from a correctly formatted USB. BIOS settings... make sure it is set to allow booting from USB and that it is the first in boot order.
The question is... what Operating System are you trying to re-install?
Comments
system model:dell latitude d420
cpu:core solo
ram:1gb
processor:genuine intel(r) cpu 1.06ghz
bios:phoenix rom bios plus version 1.10 a06
The question is... what Operating System are you trying to re-install?