reboot vista and can't find os and hard drive

when i try to boot, i get a message that it can't find the os.

if i put an xp cd in and reboot, i get a message to press any key to boot
from cd.

if i DON'T press any key, it will go to install vista screen, when i click
install a fresh windows, i realise that both my hard drive is gone.

any help would be greatly appreicated.