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Check the temps with SpeedFan ( http://www.filehippo.com/download_speedfan/ ) — you can generate a log using the configure panel, also maybe check your event log for what causes the shutdown.

It sounds like your CPU is overheating and the computer is turning itself off due to it. Maybe one of your components is attached wrongly, maybe the heatsink.

I had that the other day XD

Had the same problem. Change the power supply

and check the temperatures, I think if any part of the pc is over 100 degrees it shutsdown automatically. If its overheating, then check the pc completely from dust, and check that all the fans are running

If his processor would reach 100 degrees then the pc would turn off but he wouldnt be able to turn it on again most likely ~~ Only part of pc that can reach 100 degrees (and not break) is graphic card and most of the ones that are cappable of reaching such temperature are adjusted to it ;o My friends card, cant remember model, tough its 9 series from nvidia, is reaching 120 degrees under full load ;p

As for the topic
clear the case as first thing, especially radiator on processor. If it wont help then its 90% chance that its your PSU that is failing and in such case you can only buy new one. My advice would be, dont look at some no names and go for the quality. 450W corsair or seasonic will most likely give you better results than 600-650 W no name psu.

The last 10% chance is that your ram is failing. Run memtest to be sure (the longer it runs the better). Its highly unlikely that its ram tough, unless you get BSOD (blue screen of death) before the pc shuts down but in such case it might aswell be your graphic card memory fail.