Slave the Drive in another computer, copy over all important data. ASAP. Once done we can experiment with your Drive.
Clear CMOS, put CD Rom as First Boot device, try to boot of XP disc Again. and Run a repair.
Its possible that the HD itself is dead, either Electronically or Mechanicaly. (Which btw, if it is, its doubtful youll be able to recover Data) Report back after backing up info and the other crap I said.