After reading all the post on this subject, It looks like a failed HDD.
If you removed the HDD and booted a live CD and it worked fine, then the hdd is the problem!
I have seen hard drives fry a whole system because of a short in the hdd.
An other way to test this is to remove all other devices...cd roms and floppy drive and usb devices
from the PC and just plugin the hdd. Just the keyboard, power on and see if it will boot!
An other cause is a failing power supply...low voltage! If you have a volt meter...remove all power from devices and measure the voltage from
the yellow..12+, red..5+, orange..3.3+. 12v line is 10% + or - 5v 5% + or -.
I don't believe this is software related!