PC won’t turn on (PSU ok?)

The other day I was just surfing the web when all of a sudden my PC turned off without warning (the system had been on for some hours).
Thinking not much of it, I turned it back on again, continued using it and it has been running fine for the last few days, including running some intense programs, (rendering, etc).

Yesterday it happened again (whilst surfing), and now it won’t turn back on.
Now I’m not saying it definitely isn’t the psu, although the psu is practically brand new (see my last problem). However the LED on the motherboard and LEDs on my peripherals are all on.

If I power off and on via the switch on the back of the psu, and then press the on button, the power LED on the case will flash once, but after that – nothing.

I’ve tried unplugging all the drives from the psu to see if it will boot, but it still will not.

Any ideas?

Any strange smell associated with the systme shutting down ie burning smell? Have you checked the connections between mobo and psu? Obviously the mobo is getting some power but something could be loose. You could also try this and see if the mobo powers up without being connected Turn on PSU without motherboard - the 'paperclip trick' | Lounge | APH Networks

I tried my psu with the paper-clip test, and it appears to be dead.
I only replaced it about a month ago, maybe just bad luck I guess.
So to test it further, I got my old system out (which has been at the bottom of my wardrobe for the last 4 years, but still works), and tried the psu in that - and it didn't turn on. (the psu in the old system is too old to try in my current system.)
So is this just a spell of bad luck, or is it possible that something in my system is killing my psus?