So you have change everything and it still happens.

Have you checked the hard drive ?

I mean, you can't boot without the hard drive installed and of course it hangs before boot
if you have removed the hard drive as  the OP system is installed in the hard drive.

I wouldn't have guessed on that almost in the first case,

as since SETI/Boinc writes to the same blocks the whole time, the hard drive gets very worn at places.

You should do a hard drive test, I don't know what brand of hard drive you1 have but

they can usually be downloaded from the hard drive manufacturers homepage.

And you run it from floppy on boot, so it works even without you having boot first !

I've had about 3 like this in the past 3 months, all of which have come back to life with some bed rest--leaving it off anywhere from 90 minutes to 2 days.

I *think* what I've been doing, or rather not doing, is that I leave the power cord plugged in to have a case ground for my grounding wrist strap, but I have not been shutting off the power switch on the PSU. 

I think that may imply that the mobo is getting powered weirdly for a second whilst I put on the 20-pin connector (AMD).  Why? Because resetting CMOS didn't help, bed rest does, but the last time the fastest resurrection occurred by turning off the PSU for 30 seconds and turning it back on.

It strikes me as just the sort of stupid move that catches those of us who are otherwise absolutely sure we know what we're doing :D

ok put a win98 boot disk in and see if it boots. if it does run fdisk and see if you can see the HD.  if you cant thats your problem

if you can let me know i'll give you some other things to try

I think you need to explain exactly how long you do come.

Do you get anything when you push the start button ?

or is the screen completely black and no sounds from the PC at all ?

no leds starting up on the front of the PC ?

Or do you get a small twitch from the PSU and then it stops ?

Have you tried a Clear CMOS ?

Guys, I have exchanged bot PSU's, two Mobo's, two CPU's, five VGA (2 AGP and 3 PCI) cards in various combinations.

I have cleared the BIOS on both boards, I have tried to boot in the case and out of the case, I have tried to boot with the system in various states of construction and the same thing happens on EVERY occasion.

The PSU powers up, the Fans power up, when connected the HDD even powers up, however there is nothing else happening, the Mobo does not get to the BIOS screen, it hangs prior to this, I do not even get a BIOS flash for the VGA card that happens to be installed.

Regarding the condition of the PSU, they are both working 100%. I don't need a specific tester as I have some rather expensive test equipment as I am an Electrical Engineer..and there is nothing wrong with the power supplies. I have tested the Mobo as much as possible, and the correct voltages are going where they should.

I have tested both CPU's, all four RAM sticks and the VGA cards in this machine, they work fine..so I am at a loss.

I have external caddies for the HDD's and all work perfectly???

Ok,   just a couple wild guess' here,,   In the UK do you use 220v? and is the switch on the back of the power supply set to the appropriate voltage?

also have you checked the voltage at the wall outlet? and have you tried using a different power cord?   

Sorry dude, just trying to help.  I saw you were an Electrical Engineer in your previous post but thought I'd ask anyway.

The good News is I have found the fault and I have corrected it.

The problem was tha actual fan on the Zalman CPU cooler. It was going round as you would expect, but it was drawing too much current from the board and thus causing the problem. I discovered it when I disconnected everything electrical from the board except the CPU and one stick of RAM..she booted to BIOS!!

Thinking this odd, I then realised the only difference from other attempts was that I had not plugged the CPU fan in!!

I then decided to check this little bugger out, and it has a fault, so I have swapped the fan ion the Zalman for a spare fan of the same size I had in a box, and voila, it works fine. Now I can crunch on it again as soon as I sort out it's network issue!!