Thanks for your response. Answers/comments below:

To clairify, every time it powers off, you MUST disconnect AC cord (or
flip PSU rear switch) to get it working again, you can't just wait a few,
say 3 minutes, and it'll come on again?

it will power up again after around 5 seconds of power cord disconnection
(roughly the same time it takes the m/b led to go out)



You switched boards, so for the time being we can assume it's not the
board, but what about the OS install? Did you do a clean install, or is
it possible there's a virus jumping from drive to drive, or residing on
another system on a LAN?

Yup tried a clean install of winxp home on a freshly fdisked/formatted 8Gb
HD. It fails to get to the end of the install before powering off. This was
conducted with no other network connection or drives connected (either power
or IDE cable) so unless the virus resides in the flashable part of the bios
(extremely unlikely) there is no way a virus could be the cause of this.

What about the CPU heatsink? Is it one known to have problems, like those
TMD fans that were prone to short out? Even if another type of fan, I'd
try switching the CPU heatsink fan if you hadn't done so already.
Tried 2 heatsinks and fans. the 2100+ is a retail version and I tried the
AMD hs/f fan as well as a new hs/f bought last week. Tried on both CPUs.
Bios and PCProbe report temp and fan speed to be well within normal
parameters. Both visually look to be operating fine and there is little
evidence of excessive heat levels from touching the hs.



In theory, a virus could be attached to a motherboard BIOS, but
realistically, it would have to be a very specific targeted, engineered
attack... not a reasonable consideration for a system that still posts,
boots, and anyone with the skill to do it, could find far better ways to
spend their time destructively if that was their goal... a BIOS virus is
just not likely enough to be worth considering.
I agree


No, can't be a graphics can problem with all of 'em exhibiting the same
problem, though you probably should've tried a more modest, less power
hungry card, would isolate heat and power issues more.
agreed although, it ran fine with the 4600 for 17 months and the 9800 pro
for around a month.

"replicated with both" isn't specific.
Do you mean, you tried each DIMM separately, in each board? I'll assume
so.
Sorry, yes, both tried separately on both boards


PSU
300W unbranded, came with case

You should've never even hooked this one up to a system, especially not
with any of the above video cards. Any number of components could be
damaged because of this power supply. We can hope that didn't happen.

I would tend to agree except that it ran fine with the 9800pro (most recent
addition) for around a month during which the card received intense use (no
o/cing i might add, just lots of games).


400W high quality, multi-fan

Not to be picky, but specific make/model is more useful than telling us,
"high quality, multi fan". For example, some people think Enermax is high
quality, but might not be adequate for the parts you've mentioned above,
due to insufficient 3V/5V rail capacity. I've even seen people claiming
that trashy power supplies, like Kingwin or Turbolink, were high quality.

the 400w PSU is made by Q-Tec and is not running the monitor (separate
power). It is the recommended PSU from Maplin (a specialist UK
tech/electronics components company). I dont think i need more than 400w as
i simply do not have enough components to require that power. however, i do
accept that given the symptoms it may well be that the psu or some other
power problem is at the root of this. The chances, however, of encountering
exactly the same problem with two different PSUs seems remote.


Your voltage levels are good, checked with a voltage meter? A high 12V
rail can also indicate a problem with the 3V/5V rail that is still at
correct voltage reading.
I have yet to check voltage and would be beginning to get out of my depth.
PCprobe and the bios both reveal that voltage is within acceptable
parameters but i guess i might have missed spikes


You didn't happen to put the boards on anti-static bags? Some conduct
electricity.
no, on paper.

So the above list, is meaning that you removed ALL cards, devices, etc,
that with only the HDD, video, and a single DIMM the system still powers
off?
yes, but only in windows or during the install. it has yet to power off
whilst in the bios


Not a realistic concern, but you could always set bios to disable the
antivirus safeguard, and reflash the same bios version as it already has
(or even an earlier version) since you reported that it wouldn't accept
the latest version... which it itself is a bit puzzling unless you hadn't
disabled the BIOS virus protection and/or related jumper (I "think" those
use a bios setting, it was older boards that had a jumper, but I'm not
certain of it).
tried disabling and enabling. no change.


Any other household applicances, lights, etc, exhibiting strange
behaviour?

no. tried in two different houses too!