Is the HD set in your CMOS? Does the CMOS hold its configurations? Check ribbon cables for connectivity or swap out cables. Is it IDE or scsi? Do you have onboard controllers or cards? If cards pull and reset card? When you turn on your computer does the fan from your power supply come on or is it dead? You can also pull the HD and put it into your other computer to see if it will boot up....

Good luck,
SQ.......