PC shutting down at random moments

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My PC keeps shutting down at random moments.
It shuts down a lot when i turn it on, it will almost load up and it will shut down again, (Shut down as in all the power just goes and its off or it just restarts it self) and it can do it 3 or 4 times in a row.

If i play games ill just get started and about 5 minutes in it will just shut down.

If i try use a Virus scan, spybot scan or Ad-aware scan it shuts down.

If i use my burner it shuts down.

I cant install updates to things(like windows updates) or it shuts down

Even if i use google earth i can expect it to shut down

And when browsing the internet it has shut down on me aswell!

I dont know if its hardwear or a virus/bot/something created by the devil

I have removed the Graphic cars, The CPU and Fan, Ram Sticks, sound card and cleaned them all up and it still happens.
I have tryed taking out a ram stick thinking i might have a faulty one and it still didnt solve anything(I did it to both ram sticks.

I just dont know whats wrong and this is really ****ing me off

The only thing i can really use my computer for is for the internet but i fear having too many tabs open incase it shuts down

Please help

 

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Ive taken out all the power leads and conectors, even the hard drive ones, i cleaned them and put them back in so it cant be those.

My PC has never had any temerature guage on it and i dont know how to check where i could find a temperatures.

My PC isnt overclocked and never has been and i havnt played which any of the settings like that

One thing that has me concerned is Ebay.
When i go on that the page just keeping going down.(i think if they use a moving image because ones with a still image dont do it)


Not how far the crappy wheels have gone down on the page. They just keep on going and never stop

 

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Problemwith the disk is cant burn anything at all.

And ive tryed to scan in safemod and that shuts down too.

 

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Ive checked the ram(besides doing memtest) so i dont think its that.

Ill have to find out how i can check the temperature on my computer.
Ill be back with results later

Internet Explorer also causes problems.

Anything that puts any stress on the system at all really

 

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Ok CPU temp is around 73-75 Degrees.

The CPU shut of temperature is 85 degrees.

So thats probably whats causing it.

If i push down on the CPU fan/cooler i can drop the Temerature down to about 68 Degrees after about a minute so would it be worth trying to screw down the CPU heatsink tighter with some plastic screws to get it to put more pressure between the CPU and heat sink?
I still dont know why Ebay seems to go half way down the screen though

 

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Its just the Stock standard one that come with the Computer(Its Celleron D )

I havnt changed anything at all, i havnt even really installed any games or programs in the past month or so.