Well, despite de-dusting regularly, I am on my third power supply. Is there some form of built in redundancy? The symptom has always been the same - no "bloop" to signify that the motherboard has completed its self check shortly after switch on. The first time it happened, I was on a maintenance contract. The bloke arrived and replaced motherboard, CPU, CPU fan - and power supply. The symptom recurred about 12 months later - no maintenance contract so put in new (uprated) power supply. This lasted for a year or three then the same problem occurred - once again a new power supply from PC World effecting a cure. I don't know which supply rail fails as I have never tested the supplies. but one is certainly failing regularly with my motherboard.
P.P.