When power button clicked, flashes then turns back off

When the power button is pressed on the tower, it flashes it and the computer sounds like it about to start up when the button goes back off and doesn't start-up. Occasionally it works after end on end of pressing the button (sometimes up to 30 minutes) but crashes after 15-40 minutes. What can I do!!

[CENTER]I'm considering buying an all in one PC but I don't know if they're goof for playing games with high detail I.e Simulations. Any views?:confused:[/CENTER]
 
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