Old P4 Boot Nightmare !

Excuse the length of this post, but I have tried to cover all information so far.

My old PC, acts as a media server to other devices in the house.
A couple of weeks ago it decided it will not power on !
You get case fan run for a couple of seconds, and then it shuts down again.
It does not appear to complete the POST.

Its an old Compaq Presario S4490UK, with a socket 478 P4, 2.8 processor, and an Asus P4SD-LA mobo.

The last thing I had done to this PC is remove and re-mount the CPU fan as it was noisy. It ran ok for the rest of the day, but would not switch on the following day.

I disconnected everything from the mobo except for the CPU, RAM and fans, and it still failed to POST.
I disconnect the RAM, and you get a warning beep, but it still shuts down a few seconds later.

I exchanged the PSU, no change, so I was left with what I thought was a CPU or Mother Board fault.
I exchanged the Cmos battery to be safe, no change !

I obtained another CPU, and it done the same ! (Yes, installed with correct paste)
I have just installed another Mother Board, and it still does the same !!

What the heck am I missing here ??

Could the mobo be failing to see one of the fans run, and would that shut it down ??
(They do both run !)
The only other thing I can think off is the original fault was the mobo, and that's fried the replacement CPU.

Any other thoughts guys ??

strollin said:

When you removed/re-installed the fan/heatsink, did you apply fresh thermal grease? Whenever you re-install the heatsink it's a good idea to clean off the old grease and reapply new. It could be that your CPU is overheating and the system is shutting down to prevent damage.

Thanks.
Yes new thermal grease applied.

Will have a look for the mobo manual about clearing the cmos.

Cmos cleared, no change

TVeblen said:

When you disconnected everything down to the CPU you get a beep code. This is most likely a "no RAM detected" error code.
Now you install the RAM and it won't boot (no activity, no beeping).

This indicates that the problem is the RAM. It could be a failed stick, or it could be a failed slot. If you replaced the motherboard then that would pretty much eliminate the bad slot (still a theoretical possibility but pretty damned small!). So your diagnostic work so far indicates a bad stick of RAM.

Try running on just one stick of RAM. Then switch it out for the next stick. Do all the sticks one at a time.

When you get a beep code with only the processor installed it indicates the processor is good. You would not get the "no RAM" beep code if there was a problem with the processor.

EDIT: Obviously, your "My System Specs" are not for the problem PC. Might be helpful to know what the major components are in the problem PC should further diagnoses be necessary.

Thanks.
As said, its a Presario S4490UK.
P4 2.8 HT processor.
Mobo is an Asus P4SD-LA. Cant provide Bios at the moment.
Two IDE HDD, and one IDE dvd Drive.
Original GPU back in at the moment of an ATI 9200.
Original PSU installed, but a replacement has been tried.
Replacement GPU tried, no change.
Stripped to a bare board of PSU, CPU & Ram, no change !
Cmos cleared.
Two sticks of DDR ram. Tried with each one in turn with no change

Totally stuck now !

Just took the Ram from another PC.
Same result