"Disk Boot Failure, Insert System Disk And Press Enter"

Ok, i've just built a new system today.
i7 920 (pre overclocked to 4ghz by overclockers.co.uk
Gigabyte EX58-UD5
6gb of patriot ddr3 ram 1600 mhz
2x 4890 PCS in crossfire
1x sata f1 spinpoint 750gig HD
and an IDE optical drive.

everything is set up fine (i think) as i've built systems before.

The first boot lead me to believe everything was ok. i installed a fresh copy of vindows vista ome premium. but after expanding files etc, it needed to reboot. it did so. and then i get the message "Disk Boot Failure, Insert System Disk And Press Enter" ive checked all cable connections and all is fine.

can anyone help me with my problem?
thanks in advance

and yes. the vista disk is in the drive, and i've tried setting the primary boot to both cd and HD.. nothing.

i've already tried that, the first boot device is the optical drive, and i've tried it in both, having optical first and HDD first