is the computer passing it's POST? If not you may get some beep codes pointing to the error.
I would do one of two things or both. First go into the BIOS and set boot priority so CDROM is first. Then reboot computer with Windows XP (assuming OS is XP) is in CD and see if it will load. Alternatively, use a linux boot CD like Knoppix, and do the same thing. This will help identify whether the HDD is corrupt or not.