If the motherboard respond to start and halt signal, it means it is working, which is a good sign. I asked that, because your problem remembered a problem I had with a CUW-AM (An ASUS motherboard from HP Pavilion too). That board does not work anymore for ages now but I didn't want to bring you bad luck. The symptoms is the board starts by itself with no POST, as soon as there is power. And when I say starts, I mean the CPU fan is rotating... Apart from that, nothing gets hot, nothing lights up. I haven't been able to identify the problem then. I have no more clue today. I have a POST card for testing motherboards that does not have a segment display or PC speaker. It works with ISA and PCI ports and give information about the POST but also about the voltages. For this problematic board, I got the voltage reading, but absolutely nothing happens on the POST side. I suspect the ACPI controller to be dead.
But I would be interested to know what your P5S-VM can report, because what is strange, is that the power LED does not light up. That is what surprises me the most, 'cause if it respond to a start signal, the LED should be lit, then it could have been a CPU or RAM detection problem, so I would have given the advice to remove the RAM and the CPU, clean everything properly, plug everything back and test again. But here, I don't see where the problem is coming from... sorry.
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