Replaced motherboard, PC worked for a while, same issues again

[Hodsocks]

My second pc has been playing up on and off for some time now, some months ago it would often start up on its own, maybe boot all the way to the desktop or just stay on for a few seconds and then turn off again. I initially tried a new PSU but without success, I checked all connections and reseated memory and cards but still the same issues, eventually I swapped the mobo which appeared to have cured the problem. It worked for over 9 months and has recently started the same problem again, annoyingly often in the middle of the night. I came to use it today and it wont boot up at all, no POST nothing, no beeps from the mobo. I again reseated everything, tried a new PSU but no success. I can't say that this machine works hard because I only switch it on occasionally, often just to update the software and check its working OK. I think I used a couple of weeks ago and it was fine.
At this point in time I am a little baffled and am hoping you guys can come up with some words of wisdom.

 

[Tmagic650]

It almost has to be another motherboard failure...

 

[Hodsocks]

A rather belated update, I finally got round to replacing the motherboard and also updated the processor to a phenom II and sadly seem to be having the same problems. Sometimes it will turn on, sometimes not, when it does boot into windows after a while the screen will go blank and a message "no signal" appears on the screen, at which point I reboot it and it then often will not boot into windows. I decided to try some other memory, same problem, I tested the replacement memory and it seems to be fine. I removed the graphics card and used the onboard but still no joy. I tried another PSU thinking there may be an issue there but still no resolution.
I seem to have replaced everything except the drives, I would check the hard drives but it wont stay on long enough for me to check them. At the moment when I turn it on it wont boot to the BIOS, the hard drive light is on permanently and one of the cd drive lights is on. I would expect it to go through BIOS if a drive has gone and then throw up an error message when it tries to boot to windows.
I'm afraid my little brain is struggling to figure this one out, can any of you guys help me out?

 

[Tmagic650]

All you have left to replace is the case... Did you install Windows on a freshly formatted hard drive?

 

[Hodsocks]

The original windows installation was on a new drive, but that was 3 motherboards ago, the last two have just required re-activating windows XP. What are you thinking?

 

[Hodsocks]

Well I finally got round to replacing the case and Hey Presto everything appears to be working fine. At the moment anyway!!
I just transferred everything over without any changes to hardware or software, nought as strange as computers sometimes.

 

[bazz2004]

I'm thinking maybe the power button on the front of the old case was faulty.