As soon as you turn the computer ON, rapidly depress the F2 key and get into the BIOS. Go to the boot order and make sure that the C: drive is the first boot device.
If the problem persists, then take the hard disk out and connect it to another computer and check for errors.
If the hard disk checks out ok, then the problem is with the Nvidia chipset on the motherboard. Dell had problems with failing chipsets on the D600 series and this resulted in a class action lawsuit.
http://www.usfinancialpost.com/class-action-lawsuit-against-nvidia-chipsets-in-hp-dell-computers/85853/