No Power On PC Diagnosis help

3.5 year old Dell. Won't power on, supply fan doesn't spin. Pulled supply & it powers up outside the PC. (10 ohms between any red & black wire; green wire shorted to black for testing outside the PC). Pulled everything from the mobo (motherboard) one thing at a time, still won't power up. Tested PC power on switch with ohmmeter - tests good. No shorts at power connector pins (ohmmeter), no visible burns, no swelled lytics. Was working a week ago.
Anyone have any thoughts on what else I can check?
Thanks, Ed

ehsjr wrote:
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Note: below refers to standard psus- you don't say the model of your Dell - which may not have standard pinout/colour coding
Here's what I would do: 1) Ask at a pc hardware group with the model number - ideally a Dell one - in case the pinout or colour code is different.. 2) Disconnect all the output leads from the power supply 3) connect a dummy load (some psus wont work (reliably) without a minimum load) - I use an old 20MB (yess 20 meg) disk drive). 4) Check the mains power lead on a different appliance. 5) plug in mains power lead. 6) Check for standby 5v power between pin 9 (purple) on ATX connector and ground (any black) - no power, psu is faulty. 7) Start up main power supply by shorting pin 14 (green) and ground. 8) CHeck for volts at the disk drive connectors/other connections -
If all the above is ok - the mobo is probably dead.
If not ok - the power supply is dead.

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Reconnect switch, power supply, and motherboard. Best test is all components of the power supply system united, undisturbed, and loaded by the computer. Follow the two minute procedure posted in "When your computer dies without warning....." starting 6 Feb 2007 in the newsgroup alt.windows-xp at: http://tinyurl.com/yvf9vh
Best and informative test means power supply remains connected rest of power supply 'system' and to a full load - the computer. According to that procedure, first measurement with a 3.5 digit multimeter would be a purple wire from power supply to motherboard - both before and when power supply switch is pressed. Other important numbers are voltage on green and gray wires. And finally what happens to voltages on orange, red, and yellow wires when power switch is first pressed. Lower limit numbers are 3.23, 4.87, and 11.7. But based upon what was reported, the problem probably will be apparent long before getting to that orange, red, and yellow measurement.
Best testing does not remove anything. Reporting those numbers here may provide more useful facts that are otherwise not apparent. Chance are the problem will be apparent with the purple, green, and gray wires before and when power switch is pressed.

ehsjr wrote:
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It is now fixed. Bad motherboard. What a pain - the new mobo required a new cpu, new cooling fan/sink and new memory and it wasn't just a "drop in" replacement. Windows had to be re-installed, as the new mobo and the existing Windows on the hard drive would not work together. So I bought a new HD, installed Windows on it and copied the data over from the old HD. I spent a day reinstalling Windows, downloading and installing all the Windows updates, then reinstalling all the programs, downloading drivers & installing, etc. etc. A "nice" extra pain in the ass was that the FDD header on the new mobo did not match the floppy drive ribbon cable.
Anyway, its a much faster mobo/cpu, and now the C: drive is 320 gig SATA (vs the old 80(?) gb ide) so its a nice upgrade.
Thanks to those who replied.
Ed