i've have boards that sometimes don't pick up the delete key always. try a cold boot and tap the delete key once every second or so, starting before you ever scene the post screen.
what are you using to flash the bios? the onboard flasher or dos boot disk?
msi/delta, neo-4 //soyo/kt600-plt, kt880 //abit/kt7a-r //asrock/k7s41 //biostar/tf4u, tf6100-939 & am2
chaintech/7njs, vnf3, vnf4, 7nif4, skt600 //dfi/nf2-lp, rs482 //ecs/k7s5a-p, nf3, kn1, ka3 //foxconn/590