Disk failure

hello guys..

m having this problem for the last few days..
my pc isnt booting right..
when i switch the pc on, the first thing that comes is a blank screen with a small "dash" in the top left corner. and this thing stays for abt 4-5 mins..
then it goes to through the booting and then the error message

"no bootable device, insert bootable device, press any key.."
ive read many post and forums and one caught my eye
it said to change the ACHI to IDE but in the windows i have when i access the boot menu in the disk configuration it says ACHI/IDE - Native or legacy i have to choose between legacy and native i tryed both but to no avail..

btw i have two windows one is XP and the other is Windows 7 both on the same pc.
so now my problem is how to get access to the pc with this error haunting me

please help me m desperate all my school project works are in there and i need to get them !!!

Thank you