I had this same problem. Almost excatly like you stated Sephiroth.
But guess what I tried three different good power supplys and no luck. Guess what it was. The hard drive was faqqed some how. And yes it worked fine with another two computers. My brothers and an old pos I still have down stairs.
It did the same thing as yours is doing in setup and computer turning off. Still don't know why. But how did I fix you ask?
Well I wrote zeros to the entire drive and then it worked fine. lol Dont know why. And its still works fine right this sec. with the same computer and hardware.
But anyways my first guess for you too is the power supply. Thats always my first guess and so far it has been three out of four times. poopie, make that four out of five times. Forgot about my brother who had it happen to him a few months ago.
Oh and btw I ran dios on the hard drive and memory as well and no reported errors so, I dont always trust them. But normally they are correct. :P
So if you dont have an extra power supply just run down to a local computer shop and they should let you try another one, if their nice like me.

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