How did you test the PSU?

Does the fan on the CPU run?

Does the tray on you optical drive open?

There was a little test switch on the side of the PSU, I would hit it and the PSU would run (the fans and LED). The CPU fans would also run when I hit the test switch on the PSU. Not sure about the tray opening though, will test it out later today and get back to you.

There seems to be power throughout the whole computer, although it will not start up when the power switch is hit. Is it possibe that the wires from the switch to the motherboard have gone bad?

Any insights is greatly appreciated

It could be your RAM, try reseating it. If you have more than one module try each separately.

Can you access the BIOS?

Can you boot into safe mode?

No, the machine won't start up. But, when I hit this test switch on the side of the PSU, there is power throughout the computer, but it won't boot into startup. Ther is no POST or anything.

yea, dead mobo or psu

(psu is the cheapest thing to replace so try that first)

Don't toss the PSU just yet.
If you know how to use a volt meter try this, open the case with the power off, make sure that the on/off switch is off on the PSU, if there isn't a switch unplug the cord from the wall receptacle. Disconnect the 20/24 pin connector from the motherboard and use a piece of wire to make a jumper from the green wire to any black wire, this bypasses the motherboard. Now you are ready to turn the PSU back on, use the switch if it is there or plug in the PSU if it isn't.

For the different voltages and their color codes go to this site, the first page is the twenty pin connector pinout, the twenty four pin pinout can be accessed at the bottom of this page.

A healthy PSU will have readings that will be at least what is advertised for the rail, generally they will run a couple hundredths over.

Remember to turn off or unplug the PSU after you have finished reading the voltages.

Hi there: I was going through the posts under 'dead computer' trying to find and thank the genius who pointed me in the right direction with this very problem: my computer was running normally (3 months-old mobo, CPU - the latest gizmos-, and video card) and then it suddenly went 'phut'.
When restarted it appeared about to start up, then powered off, and subsequently cycled through this ad nauseam without ever even getting to start the POST screen. I read in other posts with growing horror the PSU (also 3 months old), CPU and mobo swap-out/buy new ones options, and I removed and firmly replaced the RAM cards, the video card and the CPU ..... no joy.
I had 4 x 1 Gb DDR II RAM cards, and I had taken out the newer pair, but not replaced them because they were hard to get in because of wiring and video card (and shoogly hands!). I then happened to read in reply to another poor soul's post with the same problem a very short but sweet suggestion: short-out and reset the CMOS.
I removed the CMOS battery, and for good measure shorted the 'clear CMOS' jumpers, put the battery back in after a few minutes, reconnected the power cable, and pressed the 'go' button .... and the computer started!! The POST went through its bits and pieces successfully, and the Widows XP welcome screen never seemed so bright and colourful!
Well, I powered down, and powered up again, holding breath all the while, and it worked perfcetly. So I thought, having removed the side panels anyway, I might as well reinstall the other 2 RAM cards. Did so, and got exactly the same nasty 'dead computer' symptoms as before .... hurriedly took the offending memory cards out, but still got the same 'dead computer' problem. So I cleared and reset CMOS again, and all was once well OK!
So be of good cheer! The memory is another possible source for this nasty problem in addition to all the other much more expensive bits and pieces admirably covered in the posts under this and similar thread, but also be aware that reolacing the errant card seemed to bugger (a nice technical term) up the CMOS when I reinstalled the faulty card/s, so that CMOS nnneded to be cleared an reset even with the 'good' memory installed.
So, to the the poster who saved me having to trek some 200 km into Tokyo for a CPU, PSU and mobo that I did not need, MANY THANKS!! I owe you several glasses of sake.