Put hdd into another computer,,,this will rule out that.
Could be mobo of course,,,esp with dial-up..

Check the hard drive settings in the BIOS. Probably will auto-detect on a
menu command if it doesn't do that already.

Did you check the Keyboard and Mouse the same way?

Its likely one of three things:
A: The power supply is shutting down because of improper regulation.
or
B:the motherboard is shutting of the power supply because it detects
an error in the power.

These will appear a bit differently depending on wether its an AT or
ATX pwr supply and case.

or
C: The mother board is fried at worst. But I would bet not. Maybe a
blown Memory Card but that would normally give you a Beep, so not likely.

I checked the problem PC using my own mouse and keyboard. We have identical
monitors so I tried both on each system. I can't fault any of her peripherals.

If you could please I need more info on the power regulation thing??
To repeat I'M NOT VERY CLEVER WITH COMPUTERS so please assume you are talking to
an idiot (probably not far from the truth LOL) and an idiot needs childishly
simple instructions.

Can you edit the BIOS?

Typically a highly charged system won't do anything much. You can check
all the systems on another computer, i.e. swap out the parts and test
them. It is hard to know what got fried and what is safe as many paths
are presented to that spike.

I'll try unplugging stuff from Motherboard and see if I can get it beeping.

It may be the power supply that took the hit. They are cheap to replace.

At this point I thought I had established that the PC had power.
I wanted to know if the MB was damaged and could communicate with a new HDD.
I changed the HDD and attempted a Ubuntu 7.10 installation.
Only seconds into the job the Ubuntu logo went out and my monitor was filled
with multicolour (very colourful!) "snow" and this is as far as I could go. I
shut down and repeated several times with the same result.
I have an installation disc for Ubuntu 5.04 so I tried to install that and it
seemed successful.
Can you or anybody suggest why 5.04 would install and 7.10 would not?
My aim is to have 7.10 fully working as it used to.

With my LIMITED pc surgery experience I'm kinda intimidated at the thought of
plugging in borrowed power supplies, gulp

My own PC has a ATX 250 watt and the problem PC has a ATX 350 watt.
Seriously can I do harm by just plugging my working PC power supply into the
other one?
I only have the two PC's here and whilst I have a few spare HDD's and some other
stuff spare power supplies I don't got (never changed one either)
By the way my last test was to try to install XUBUNTU 7.10 over the top of
Ubuntu 5.04 and the same result as with Ubuntu 7.10, the monitor went to
coloured snow 15 seconds into the install.

it will not hurt nothing to test power supplies

UPDATE
The owner of this old problem PC has decided its caused enough hassles and she
was thinking of buying a new one anyway.
So I've been asked to pass on her gratitude to all the members who contributed
to this issue.

can i ask why u are still using 7.10 cause it is not supported any more just
wondering

My PC is maybe 7 years old, about the same as the one I'm trying to fix. Very
similar specs. P4 2.6 , 512mb, 10gb HDD
I first tried Linux a few years ago and at the time Ubuntu 7.10 was current and
after some brilliant help from this group (Joe in particular, thanks pal ;) I
was happily up and running with dial up internet.
The thinking at the time was that my "OLD" system was not going to be compatible
with future upgrades and that the older PC's are better suited to the older
versions.
It was suggested that 8.04 was the most modern version I should attempt to
install. By the time I got this advice I could no longer get 8.04 easily so I
have made do with 7.10 and I have to say it has served my VERY simple computing
needs well.

8.04 is still avaiable for download on website

I have a dial up connection isn't that gonna take forever??
I bought my Linux CD's off Ebay (I think they were a few bucks) so I got a
properly burned live copy but the folks who sell these CD's only sell the latest
distros as a rule and I was too slow to get 8.04

If one version installs and the other goes TU in a big
way I would guess a bad video card. Probably wouldn't
hurt to run a RAM test while you're at it.

This box has an all in one MB and I don't have a video card lying around I can
slip in so can you think of a SIMPLE test I could run using the installed 5.04
that will confirm if the video is faulty?
I did run a memory test and it checks out OK.
Please excuse my ignorance, I'm quite out of my depth on anything but the most
basic of PC surgery.

UPDATE..........
I dunno if you think this is a proper test of the video capacity of the suspect
MB or not but I borrowed a CD with high resolution colour images on it. I better
not say what the images were of but it was my kind of video test LOL
I slipped the CD into the box and had no trouble getting 5.04 to display these
lovely images in glorious living colour ;)
I even got carried away and had several open at once..........
I'd guess that this test was putting WAY more stress on me, er, I mean the video
card than was the opening image of the Ubuntu 7.10 installation.
What you think??

Might be getting somewhere thanks to you guys :)
I unplugged the HDD and FDD from MB , left the DVD rom plugged in and powered up
the box.
I got a single beep (no beeps previously), keyboard led's blinked this time and
the monitor sprang to life :)
It could not boot without an operating system so I shut it down and rebooted
this time with my Ubuntu 7.10 live CD in the DVD rom.
I did a memory check using Ubuntu in live mode and so I figure that the
motherboard isn't totally fried if it will do this much.
I have a couple of spare hard drives here so I will swap a good one in and see
if that fixes the problem.