[rhx123]

I have a rather large problem with my computer and wondered if anyone else had an opinion on it.
My computer is turning itself off during the OS loading. As in, if the power button has been held in.
I have changed all the hardwear round bar the processor and motherboard and it carries on happening. It's frustratingly hard to troubleshoot as sometimes (1/5 times) it starts up just fine.
It does this both under windows 7 and linux.
I put my old XP hard drive in and again it crashes during startup.
Once the computer has made it to OS (as in the select user screen on windows and ubuntu) the system runs perfectly stably.
This is a list of things I have tried:
-Swapped ram
-Remove soundcard
-Taken out graphics card and used onboard
-Swapped PSU
-Taken board out of case and ran on desk
-Reset bios to factory settings

Does anyone have any ideas or should I just put it down to a processor/motherboard problem.

[Nuke]

make sure the wiring in your front panel is good. ive had at least one computer where the fp wiring was bad and the computer would intermittently reset. this case im using also has a power button that likes to stick, sometimes it doesnt pop back out causing the system to eventually hard shutdown. a short in either the power button or reset button lines can cause intermittent shutdowns.

you may also want to check your power, sometimes noisy power can cause these kinds of issues. try a line conditioner or ups if this is the case. this can actually be a pretty troublesome problem to solve. i once installed a network of computers into a 100+ year old building and had all kinds of issues like this, and a lot of the machines were just flat out fried.

other things to check, run memtest, make sure your memory checks out. even new ram can be bad or setup incorrectly. make sure your psus put out enough power, make sure connectors are seated properly. make sure its not an overheat issue. some mobos auto shutdown if they detect an overheat.

[rhx123]

Cheers for your reply
As I said in my previous post, I have taken the board out of the case (I booted it up using a screwdriver to short the power terminals) so I know it isn't the fp wiring.

Memtest is fine, and my psu has been fine for 4 years in this system now, its 450W and its a athlon x2  4000+ and A 9600gt, neither of which are particualrlary power hungry.
I really doubt its an overheating problem, as the heatsink on the processor isn't warm to the touch, as this happens on a cold boot. Also as I said earlier, on the 1/5 times it does work, I can play games fine with no instability whatsoever.
I will try a power conditioner I have about, but again if it was a problem with power I'm not sure why it happens so predictably at the same time during boot.

I've worked as an IT Tech and have delt with lots of computers, and whilst i've seen similar problems its usually been faulty memory.

Again thanks for your help, I may try disabling some of the on-board devices on the mobo to see if that makes any difference.
And btw, the entire system is stock (I built it) but the clock rates of anything have never been touched.

[Klaustrophobia]

[shot in the dark]

bad CMOS perhaps?  try flashing bios? 

[rhx123]

Yeah i'll give that ago, but it seems strange for it just to stop working after it's been working fine for 4ish years now.
I have a spare board about that supports core2's so it might be time for an upgrade anyway.

[jr2]

Hmm,

not sure, but it could be possibly a bad firmware in one of your devices on the motherboard?  (The BIOS of many devices {that gets called by the mobo BIOS during POST} have "drivers" of a basic sort that load until the specialized Windows / Linux / Mac / other OS drivers take over.)

Yeah, do they have any motherboard BIOS updates also, check that, maybe there's a weird bug in yours.