Sudden Power Off [Fingers Crossed!]

[#1]

All seemed to be OK with my PC after I powered it up at about 16:30 today.
Then at about 17:50, it suddenly powered off without warning. Nothing connected to it lost power.
I powered it straight back on OK. I thought it might have been overheating, but I ran Speedfan on the CPU and it stayed steady at 43 degrees.
Then at about 19:40 the same thing happened again.
I took the covers off to get a good look. Nothing seemed to be untoward, but I gave it a bit of a clean anyway.
When I went to power it back up, the fans (PSU, case, CPU and graphics card) powered up fine; the lights on the multi-card reader came on, and the HDD seemed to be OK too...but there was no POST and no signal to the monitor.
I checked the connections from the PSU to the MB and HDD, cleaning and reseating.
I just powered it up again, and it booted into XP fine. I was just about to go back online when it powered off again.
Any thoughts about what's doing it? PSU on the way out? The machine is about 4.5 years old. XP Pro SP3. Athlon 64x2 dual-core chip.
TIA

[#2]

There's a vague possibility that it might be caused by a virus/trojan.  It;ll be worthwhile running a scan to eliminate it.
You should also check the event log to see if anything has been reported in there that may give a clue,  but it's rare to find anything useful.
Another check  worth doing, when powering up, go into the BIOS and check the hardware (health) status,  that should show a few temperatures and also the voltages from the PSU.
However,  my main inclination would be a failing PSU,  as ever, it'll be worth swapping it with a spare (if available) to be sure.
I've also had a case where the graphics card failed,  in that instance the machine would not POST with the card connected;  it's possible yours may be on the way out.

[#3]

I've gone into the 'PC Health' section of the BIOS, and it's showing a clean bill of health..
Here's an oddity: after I posted last night, I booted the machine up again and left it at the login screen for about 10-15 minutes. No power-off.
I logged into my second account and browsed for a bit on that. No power-off.
This morning (i.e., now), I left it at the login screen for about 15 minutes before logging in to the second account. 25 minutes so far and no issues (yeah, I know, Famous Last Words!).
Could my main profile have become corrupted, and that's what might be causing the problem? I haven't tried the main profile again this morning (don't have the time), but is this possible?

[#4]

It's possible that one of the programs that runs when you login is causing the problem.
You'll want to check the various user specific places to see what's loading at login,  Autoruns is a good tool for the job.

[#5]

It's the heat.
Maybe. 

[#6]

Got home Thursday afternoon, booted up and left it at the login screen for about an hour. No problems, so I logged into the unaffected profile and used that for about 4 hours (including running a full malware scan, with resultant thrashing of HDD - if there had been a power issue, I'd have thought that it would have manifested itself then). Again, no problems.
Finally, set up a whole new profile, and copied all of the dodgy one across except the 'ntuser' files (i.e., the registry).
Two hours running on that last night, about four hours today, and so far no issues at all.

[#7]

Sigh...
Came to switch it on this morning - fans powered up, etc. etc., but wouldn't POST.
Got back on my backup rig and did some research. Tried one of the ideas I found - disconnected everything from the case, held the power button in for 30 seconds, then powered it up with nothing connected. Went to POST and booted up fine. Powered off, reconnected everything and powered up again - fine.
That was about 5 hours agp. So far, so good. But I'm going to buy a new PSU this week just to be ready...

[#8]

Here's the latest:
Everything was OK all day Saturday. Machine POSTed and booted fine Sunday and worked OK. Same thing this morning before I went to work.
Tried it just now, and it wouldn't POST again. HDD spins, fans spin, but no light on the multi-card reader and nothing more going on.
I said some naughty words...again.
I unplugged the power from the back and left it while I fired up my old Win98 rig to do some research. After about 20 minutes, came back to my problem child, held the power button in for 30 seconds again, then put the power cord back in and fired it up. This time it went to POST and booted up normally.
Am I looking at a dying PSU here, do you think?

[#9]

I had the same problem with my parent's PC (5 year old Athlon64) - it powered on to a blank screen, no POST, hard disk spun up, fans on.
Since there was a spare motherboard + CPU + RAM available (a similar Athlon64 but in a different socket and DDR2 instead of DDR ram), that lot got replaced, and it currently works.

[#10]

Yes, PSU, MoBo, CPU can all cause exactly this symptom, possibly also graphics card, less likely memory or HDD.
Systematic swapping of components is the only way to be 100% sure of the culprit though sometimes just disconnecting all power connectors (especially to the MotherBoard) and re-connecting them can solve the problem.
I don't suppose the PSU from your old W98 rig has all the correct connectors required to test on the suspect one?

[#11]

A friend had a similar problem why I tried to diagnose and after finding that it still did it with a linux boot disc I blamed the motherboard and suggested he got an expert involved.
After a number of tests the specialist found that the graphics card was fried so he put a new one in - and it fried that as well. 
As the replacement cost of the motherboard was unreasonable he fitted a modern motherboard with modern memory and integral graphics for a lower cost. 
The point of all this rambling was that the motherboard seems to have been creating a short somewhere which tripped the over current protection of one of the outputs of the PSU and eventually fried the graphics card when the current trip didn't come in fast enough

[#12]

Trouble is, it's one of them there annoying intermittent faults.
What I'll do tomorrow evening is open it up, disconnect and reconnect whatever I can and then see what happens.

[#13]

I don't recall ever coming across an erratic problem like this that was down to the PSU.  They tend to fail once and for all.  My bet would be the motherboard but, unfortunately, you can only prove this by eliminating everything else and your failures are so infrequent that doing so could take weeks.

[#14]

Have you considered a dying (BIOS) battery?

[#15]

A dying BIOS battery normally shows with erratic performance at boot up, but worth changing as these batteries only costs a pound or so. My money is on an ageing component on the motherboard which is borderline on the edge of total failure. You might be able to find a replacement mother board of the same type, failing that a new mother board and processor bundle is the most likely fix, but often you end up changing the memory and disks as well.  MISCO are doing some refurb'ed PC's  for between £80 - 160 at the moment.