system black out and restart under varying situations

Well, you need more memory (I recommend 1Gb for XP and single core CPUs) to make XP happy and run better, and reduce the wear and tear on the hard drive, but that will not fix this problem, which is not related to how much memory you have. Sudden reboots can be caused by many things:

Malware - have you done a thorough scan?
Heat - what are your critical temperatures (CPU, System, GPU)? Is the interior of the case clean of heat trapping dust and dirt?
Weak or failing power supply - see canned text below for testing PSUs
Failing RAM - see canned text below for testing RAM
Leaky capacitors - see canned text below for inspecting the motherboard for leaky caps
Failing motherboard - no test for that

To properly and conclusively test a power supply unit (PSU), it must be tested under various realistic "loads" then analyzed for excessive [url="http://en.wikipedia.org/wiki/Ripple_(electrical)""]ripple[/url] and other anomalies. This is done by a qualified technician using an oscilloscope or power analyzer - sophisticated (and expensive) electronic test equipment requiring special training to operate, and a basic knowledge of electronics theory to understand the results. Therefore, conclusively testing a power supply is done in properly equipped electronic repair facilities.

Fortunately, there are other options that are almost as good. I keep a FrozenCPU Ultimate PSU Tester in my tool bag when I am "in the field" and don't have a good spare power supply to swap in. While not a certain test, they are better than nothing. The advantage of this model is that it has an LCD readout of the voltage. With an actual voltage readout, you have a better chance of detecting a "failing" PSU, or one barely within specified ATX Form Factor Standard tolerances. Lesser models use LEDs to indicate the voltage is just within some "range". These are less informative, considerably cheaper, but still useful for detecting PSUs that have already "failed". Newegg has several testers to choose from. All these testers contain a "dummy load" to fool the PSU into thinking it is connected to a motherboard, and therefore allows the PSU to power on, if able, without being attached to a motherboard - great for testing fans, but again, it is not a true load or suitable for conclusive testing.

As mentioned, swapping in a known good supply is a tried and trued method of troubleshooting used for centuries, even by pros. Remove the "suspect" part and replace with a "known good" part and see if the problem goes away.

I do not recommend using a multimeter to test power supplies. To do it properly, that is, under a realistic load, the voltages on all the pins must be measured while the PSU is attached to the motherboard and the computer powered on. This requires poking (with some considerable force) two hard and sharp, highly conductive meter probes into the main power connector, deep in the heart of the computer. One tiny slip can destroy the motherboard, and everything plugged into it. It is not worth the risk considering most multimeters, like plug-in testers, do not measure, or reveal any unwanted and potentially disruptive AC components to the DC voltages.

And remember, anything that plugs into the wall can kill. Do not open the power supply's case unless you are a qualified electronics technician. There are NO user serviceable parts inside a power supply.
***

You can test RAM using one of the following programs. Both require you to create and boot to a bootable floppy disk or CD to run the diagnostics. Using the floppy method is generally easier (and another reason to include floppy drives in new builds). However, the CD method is just as effective at detecting RAM problems. Allow the diagnostics to run for several passes or even overnight. You should have no reported errors.

Windows Memory Diagnostic - see the easy to follow instructions under Quick Start Information.
or
MemTest86+ (for more advanced users) - an excellent how-to guide is available here.

Alternatively, you could install a single RAM module and try running with that to see if it fails. Repeat process with remaining modules, hopefully identifying the bad stick through a process of elimination.
***

Inspect the motherboard for bulging or leaking electrolytic capacitors. These failed or failing capacitors are a common cause of sudden, but seemingly random system lock ups and reboots. The capacitors look like tall soda cans, many of which surround the CPU socket.

All older motherboards, and many of today's less expensive motherboards use electrolytic capacitors containing a liquid electrolyte. Failing (including flawed and/or abused/over-heated) capacitors literally bulge at the seams due to excessive internal pressures. Extreme (and very rare) cases result in a firecracker type explosion that can really stink up a room. Typically, electrolyte just oozes from the pressure relief point, which appears as a symbol or letter stamped in the top of the capacitor casing. The electrolyte can be caustic to motherboards and flesh. Look for white to dark-brown, dried liquid or foam on the tops or bottoms of the capacitors. Bulging capacitors are a sign leakage is about to occur.

A motherboard with bulging or leaky capacitors can be repaired, but often it is more cost effective in the long run to replace the motherboard.

Be sure to first power down, unplug the computer, and keep yourself discharged by touching the bare metal of the case before reaching in.

Since original post I have checked my temps. I find the Main bd is at 47C Cpu 70C,Temp3 37C,HDD 32C, my 1 fan runs betwenn 2481 & 2556. PS voltages are + 12v=11.67,+ 5v=4/92, core+ 1.49,+3.2=3.2
- 12v=12.12,- 5v=o Aux = 1.50

I am giving the figures but have no idea if they reflect any problem with the PS. The main problem at this point is still the crash and restart at the desktop and then everything is back to normal. This crash/restart problem seems to be when typing an email message or reply to one. I also happens when typing on word. Since we were losing email replies before posting, very frustrating. That was why we went to word, saving as we go along and then pasting etc to our email. The only time I recently had the problem was a few days ago after downloading google earth and just as I had found the location in London and was honing in, the crash/restart happened, although I have gone to Google/Earth with no problem. Come to think of it, I have been typing this and no crash/restart so far. If it happened in word I would blame the email site but it has happened both on AOL and hotmail.

I printed out your original response and read it closely. I just had the bios battery replaced and I knew that wasn't the answer beforehand. I have checked with Crucial and a 1G Ram would only cost $42-$43. But you indicated in your first response that wouldn't solve the problem. Hopefuly this info might give some add'l info which could be of help.
Thanks in advance for any help/info you can supply......URBROJR

Just to clarify, in reference to your RAM, I was saying that having only 512Mb of RAM will not cause your system to crash like this, so adding more RAM won't fix it. If your current RAM is failing, then of course, replacing it would be in order.

70°C is normally too high for a CPU. Your voltages are fine - ignore the -5V readings, that voltage is not used. And as noted above, just ensuring the voltages are correct does not prove the PSU is not unstable.