If the POST sequence is showing up but you get nothing but a black screen afterward, then the hard drive is not communicating with the system. Are you seeing any error messages at all after POST? Have you tried opening the case and reseating all cabling to the hard drive, such as the power and ribbon/SATA cables? If that doesn't help, then you're next step is to attempt a repair install with the XP disc.. If it doesn't help, then it's time for a new hard drive.

Hope this helps.