bad power supply?

Not necessarily.

The power supply needs a minimum load to power up. If the load is too small, the power supply will not start.

While the power supply is the likely culprit, the motherboard is also a candidate.

Check that the processor and memory modules are seated properly, and that the CPU fan is plugged in.
Have the video card, and at least one drive connected.
If there is a power switch on the back of the power supply, switch it to off before plugging in the system.
Also, check the 120/220 volt switch on the power supply.
Switch the power switch to on.
While watching the CPU fan, press the power button.
If the CPU fan doesn't spin, a safety feature may be what's shutting down the system.

If the CPU fan starts to spin,and the system still shuts down, I would go through the proceedure to reset the CMOS memory for the system.
If necessary, shut off the power switch, (or pull the plug) and remove the cmos backup battery for 30 minutes or more.

If this doesn't bring it back, then I would contact the mfg for return instructions. I wouldn't start swapping out parts without their approval, or you may void any warranty.

Chas

Determining if a Power Supply is bad or not is difficult, does a constanthigh pitch tone come from the computer,it needs to be checked with a volt meter to determine if their is adequate voltage.

Check the memory chips,by uninstalling them and reseating to see if this is part of the problem

Try to determine if the one beep is not the post beep that signals the computer is booting

The motherboard could be the problem.
Also a determining factor of power supply not booting would be the hardware that it needs to supply power to,a loaded pc with dvd,cd/rw/ etc will pull a power supply down.

I've just gone through the rounds with 3 different cases where the MB failed to boot.

For all 3 cases, the spacers for mounting the Motherboard were too short, and the AGP card would not seat all th eway into the port.

To check this, remove the bracket from the video card, and insert the card into the socket. If it seats further, the standoffs are too short.

On another board, the AGP connector itself was bad.
I was finnally able to get the board to boot with a PCI video card.

I also have discovvered that some socket A motherboards do not set the CPU frequency properly, and need to have the CPU clock set manually.
You may need to go through the clear CMOS proceedure in order to get the board to boot the first time so that youcan set the CPU clock.

Chas