Steevo: I checked the caps one by one. They appear proper and neat like new caps should be. No bulging tops, no abnormality at the bottoms, no leakages. I even sprayed a special coolant specifically to locate cracks on electronic boards just in case, and I found no cracks on the pcb.
Silverel: I hadn't thought about the mobo battery since my date and time had been working perfectly before the start of all this muck-up. I have a couple of new and newish batteries lying around. I'll test them out after I finish the second attempt at a fresh re-install (it's around half-way now).
Edit: Come to think of it I did have that brand-new syndrome. Previously when my computer was at least loading windows, at random I'd find programs which had been installed a long time ago marked up as new. You know how when you press the Start button and select 'All Programs' the new ones would be highlighted? I was getting like 75% of the stuff there highlighted as new, even though it wasn't and I had accessed those programs before.
2nd Edit: I managed to get Windows to install properly. I don't think it's the mobo battery.
Whatever it is, it's something which is getting either hot or humid (probably caused by corrosion). I can say that the cpu temperature is OK, and so is the FX5500 (I had same problems with onboard graphics), the HDD can't be the problem because I'm using a new one.
I am now sure it's a hot/humid component because I placed the tower on the desk, with both sides open and with a very powerful house fan (the high-rev ones with metal blades) blowing at it.
Windows installation went flawlessly. Pc is functioning flawlessly.
Now we'll see what happens when I unplug the 23" fan......
Edit 3: As I said XP fresh install went flawlessly... only to discover I got no mouse pointer. I could navigate through windows limitedly using the keyboard.
I went into the bios and made sure the settings were still correct. Still nothing.
I plugged a PS2 mouse, just in case (PS2 port had called it quitters some years ago). Still nothing.
Rebooted.
Keyboard doesn't work now.
Rebooted like 6 times, in vain.
While POSTing the keyboard LED's light up but nothing else functions. I can't even access the bios now.
Apparently Steevo has got a good point. I'm sure that there is a capacitor or maybe some other component in the motherboard which is giving up. I think I managed to install XP only thanks to the freezing blowing wind of the fan...
So now I have a pc which at least doesn't keep rebooting by itself before loading XP. XP actually loads but I have no mouse function, no keyboard function, no bios access.