Computer is shutting down by itself

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My windows XP computer has been shutting down by itself for about two weeks now. It is intermittent in doing this. Does anyone have any suggestions on what could be causing this. The computer is in a clean area of the house, no cats around, and not much foot traffice moving arounnd except when I come upstairs two or three times a day to use it.Any other suggestions. I also believe I am clean of virus according to my MSE. 

Any idea?

Please help.

Thanks!

I didn't find the right solution from the Internet.

References www.bleepingcomputer.com/forums/t/528394/computer-is-shutting-down-by-itself/

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