Trying to help a friend, so don't have complete details . . .
PC had been sometimes failing to boot. By the time I was called in, the CMOS had been cleared.
We tried to boot with minimal hardware, nothing plugged in except CPU and RAM, and it halts during the POST at step 13 (D-Bracket LEDs 1, 3 and 4 green, "Initializing Floppy Drive Controller") and then, after a few seconds, it powers itself off.
This happens with both DIMMs installed, or with either one alone.
(I'm told that, earlier, it did not turn itself off, but instead it would reach a certain step in the POST and then start over at step 0, as if the reset button had been pressed.)
When we install the floppy drive, we do hear the normal buzz-buzz-buzz, but otherwise it's exactly the same.
When we install a graphics card, it's exactly the same. (Tried two cards, the PCI Express x16 nVidia card he had been using, and also a brand new, low-end, "regular" PCI card.) At some point, the monitor LED goes from amber (stand-by) to green (on), so it must be receiving some sort of signal, however nothing appears on the screen, not even the video BIOS message.