My PC has a mind of its own

I am trying to start up my computer and every time i turn it on it starts up for like 3 seconds, then turns off then trys to start it self again. What is wrong with my pc and how can i fix it?
 
Could you give some more details? The brand, model number, specs, ext. It sounds like a motherboard problem to me, but more info would be helpful.
 
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