PC sudden "shutdown" or power off

It is likely your power supply that is malfunctioning.  Swap another one in that is just as powerful (don't skimp on quality, because a failing power supply can cause all kinds of damage).

Like Callandor said buy a quality PS like Antec or Enermax. You might also check the PS  connections to the MB.  

You may also want to check your bios settings for automatic shutdown circumstances.  Like times / dates,  heat, fan-speed, voltages...etc.  Alot of newer bios's have advanced features to protect your system and shut it down in the event of some type of failure.  If your motherboard supports monitored fans, make sure the PS, CPU, and Northbridge fan are pluged into the correct places.  That the processor heatsink is installed correctly with an air-tight connection between the heatsink face and the processor using something like Arctic Silver 2.

As stated above, most of these problems are caused by bad power supplies, I reviewed the super flower 450, and it seems upon first glance to be a decent power supply for someone into LED lights and things, but the guts of it seem quality enough, so it's possible you just got a bad one.  You really could easily run a 500 to 550w power supply under a high heat peak load for the system you spec'ed.  Two hard drives, two cd-rom drives, you didnt mention video card which is either the single, or the second biggest draw of power in your system, sound card, network card, modem or otherwise, but I assume you have atleast 2 of the 3 installed which draws power.  Remember the 4 fans on your powersupply all suck juice, as well as your processor fan, case fan ( x 2?) northbridge fan, video card fan.

Here's how it breaks down.
http://www.pcpowerandcooling.com/maxpc/index_cases.htm

Add up your systems wattage, figuring the worst case scenario, where everything would be on at once.  Then buy the best branded ps you can for your systems consumption.  As mentioned above.  Antec or Enermax is by far the best choices out there.  Having icky power supplys has never caused me anything but problems in 12 years of building systems.

could easily be due to cpu or memory failure too, although usually those cause a bluescreen before dying.  usually a power supply failure will be most noticable by smoke or some stench in the air around the power supply.

- Jer

Try checking the CPU temperature before chainging the PS. It might be that it's temp. increases to the limit specified in the BIOS. This should cause the PC to shutdown to protect the processor.

As for the computer automatically starting up when you switch the main power on, check the BIOS for the option "Power on after power fail". Turn it to "Off" if you don't want the computer to start when you turn on the main power switch.

                             When it shuts down does it give you the whole "windows is shutting down" deal or does it just turn off abruptly? if it's the "windows is shutting down", it could be a virus of some sort (which would be weird if you've done a couple re-installs). if it just shuts down abruptly, it could be a lot of things.
                              I would look specificlly at either your motherboard or your powersupply. if your powersupply is providing bad power it could easily affect your system state. if you have a multimeter (voltmeter) you can test to see if your power supply is providing proper power (try this only if you know how to use a multimeter and are aware of the proper safty precautions needed).
                              For this issue I would suggest you to check and replace your power cables and if needed replace everything that is involved in power supply as quickly as possible, as this could even damage your monitor.

well your problem is about 60% over heating of CPU this is the first thing you should check out.  Did you put the grease on when you put on the HSF? your other 40% is split between a bad power supply or not enough power on the outlet.  The reason I suspect it to be CPU over heating is because it stayed on longer when u let it cool down a bit.  Power supply could also be an issue if it has less voltage then your system needs or of bad quality.  Lastly since u mention ur monitor flickoring it could also mean that on the outlet too much devices and not enough power, but that would be not very likly but possible.

If  the system Shuts down and Blinks amber it could be the Power Supply. If the Power LED is not blinking Amberand if you are not able to Power on the Machine Straightway after the shutdown, its could be the CPU fan Failure or Processor over heating. Check the Cabling to the Fan and check if there is some compromised performance. Ensure that the Heat Sink is firmly attached. Stay in Bios and see if the system Shuts down in bios or not. This can be done to isolate Software induced Shutdowns. If the system Still shuts down in the Bios then Definitely you have a Hardware problem.

Hope this Helps

Cheers
Sinu.

Thanks for all the experts for ur comments. The problem is due to faulty hard disk.

Hi, I cant find any link to delete or close this question. Can anyone p/s help me to close it or share with me how deletion is done?

This is from a faulty hard disk?? Hmm, thats a new one on me.
But if it solved your problem I suppose thats good news.

Post a note in Community Support with a link to this question, and someone will direct you as to what you can do.