Opti 755 dead - power supply or motherboard?

Today, my Dell Optiplex 755 Desktop (Win XP Pro SP3) went completely dead. When I plug in the AC, there's no spin-up or short fan burst. When I hit the power button, nothing happens. The power button has NO light - not yellow or anything, just dark. No beeps. Silence.

The machine was working this morning, and I went to turn it off to look inside because I'm thinking about adding a hard drive (which I have NOT yet done).

But maybe I did something when I turned off the machine, pulled the AC plug, then hit the on button to drain the capacitor. My finger slipped, so I hit the on button again. Maybe that did something.

In any case, after looking at the insides of the machine (and doing nothing), the machine would NOT power up again. Dead parrot EXCEPT that the little LED on the motherboard near the unused Aux Power socket comes on. But nothing else.

I decided to check 100% whether or not it is the power supply, so I bought a multimeter, and here are some interesting preliminary results:

P1 is the big rectangular plug with 24 pins in two rows that goes from the power supply to a big socket on the motherboard. According to the Dell manual for this Opti 755 Desktop, many of the pins should have 3.3, 5 or 12 volts DC. Well, they're all 0 zero except for:
- Pin 9, which has 5 volts (good per manual) and is described as P5AUX in the manual, and
- Pin 16, which has 3.5 volts (manual doesn't say what voltage should be) and is described as PWR_PS_ON* in the manual.

P2 is a square plug with 4 pins in two rows that goes from the power supply to a socket on the motherboard near the front of the machine. According to the manual, two of the pins should have 12 volts DC. But they are both zero.

P3 is a rectangular plug with 4 pins in one row that I have never used - from power supply to nothing. Two of the pins should have either 5 or 12 V DC, but they are both zero.

I cannot check P4 or the two SATA power connectors to the hard drive or DVD drive because the holes are too small.

SO - what do you think? Do I have a bad power supply, or is this normal because the machine first needs to boot up in order for these other pins to get juice?