This is a custom machine that had been working for years for my friend. He said it froze and he unplugged and replugged it into the wall outlet a few times. After that he called me.

The machine has no PCI, Video or any other cards in it. Just the Motherboard, CPU, Heatsink/fan, and memory stick. (I unplugged the SATA Hard Disk). It has Video on the Motherboard.

It just plain will not POST any more. I first replaced the Power Supply...no POST. Second, I replaced the Motherboard (Intel D915GAG) with a new D915GAG motherboard and the original CPU (P4 3.0GHz)....still no POST. Third, I replaced the CPU with a P4 2.8GHz....still no POST. I tried replacing the memory with no luck. I connected a old IDE Hard disk but it does not spin-up when the power is turned on....Why is that?

Interestingly, the "Standby Power LED" on both the new and old motherboard is lit when the power is turned on indicating that at least some power is getting to the motherboard.

I am at a loss as to what else to try. Any ideas?

try a new or known fully working power supply

try a new or known fully working power supply

I forgot to mention the power supply I tried was brand new out of the box.

Any other ideas?

try doing it on the bench and not in the case ,to make sure its not shorting out in the tower

try doing it on the bench and not in the case ,to make sure its not shorting out in the tower

I tried the MB outside the case. I sat the MB on a piece of cardboard and isolated it from any other contact. I connected the power supply, keyboard, mouse, monitor, 1MB RAM, and a IDE Harddrive.
Still Dead! The "Standby LED" is lit on the board indicating it is getting some power of some kind but it does not spin-up the hard disk or show any other indication of life.

Have you tried different sticks of RAM in each of the dimm slots?
I think it very strange that you have changed everything, yet still have the existing problem.
Try resetting the cmos with no power to the board.

How are you turning the power on when the motherboard is outside the case? Do you have a power switch connected to the motherboard at the proper pins? If you changed everything and the computer will not post but you do show standby power (as indicated by the LED on the motherboard) then it would seem that one of the connecting cables for the power switch or the power supply is not connected properly.

Have you tried different sticks of RAM in each of the dimm slots?
I think it very strange that you have changed everything, yet still have the existing problem.
Try resetting the cmos with no power to the board.

Crunchie:
Thanks for your ideas!
Yes, I have tried different RAM Sticks in different slots, and
Yes, I reset the CMOS.

How are you turning the power on when the motherboard is outside the case? Do you have a power switch connected to the motherboard at the proper pins? If you changed everything and the computer will not post but you do show standby power (as indicated by the LED on the motherboard) then it would seem that one of the connecting cables for the power switch or the power supply is not connected properly.

BuggZ:
Thanks for your suggestions!
I jumpered the green wire and a black wire on the main connector (P1) to turn on the power supply....I connected a case fan and a hard disk -- the case fan and hard disk spin up.

I have checked the power switch connection several times and also had another person check my connections again.

I think it has got to be the CPU.

I took the machine to a PC Repair store. They determined that the NEW Motherboard was defective. I don't know if the original CPU is good or bad but the replacement CPU that I purchased is good and the new Power Supply is good. I am returning the defective new motherboard but have not decided to try another Intel D915GAG motherboard or just start over with a newer technology motherboard/CPU/memory for this case.