strange hard disk detection problem

hi i m getting a strange problem in my pc. It
detects my hard disk alternately means one
time it detects the hard disk and loads the XP
but if i restart it gives error "Reboot and select
proper boot device Insert boot media in
selected boot device". From here if i again
restart OS loads. I can't understand the
problem. Checked the Bios once the hard disk
is enabled and once its automatically gets
disabled we need to reenable it, if we want to
load XP each time. The board is an Intel 845.
Pls reply its very urgent