Re: system shut down
I had the same problem, end result was that it was caused by a new WD hard drive.
When the HDD was plugged in computer would shut down with out a warning just turn off like someone unplugged power from it.
PSU was fine, because in place of that WD, i plugged in 4 other HDD to stress it a bit, and computer was working perfectly fine. Put the WD back, and few minutes later its off.
I would recommend testing each component, only plug in what its absolutly required to start the computer, nothing else. If it still shut down, try to change the HDD to a different one.
Windows 8 the next big failure, right after Windows ME