This problem typically crops up on older Dell Dimension and Optiplex desktops but could also happen on the Inspiron and Latitude laptops. It seems scary because it acts like the hard drive is dying but usually its a lot simpler than that!
Boot the computer and immediately start tapping the F2 Key to get into the BIOS. Turn on Caps Lock, Scroll Lock and Num Lock. Hold down the ALT key and then press (in sequence) E, then F, then B. Do not hold them down and do not press them at the same time. The computer will reboot and reconfigure IDE. That should solve the problem and the computer will boot normally.