PC won't power up

If this was a "simple" fix for the other system, you likely performed a reset on the CMOS memory to set the BIOS option settings back to factory default.

Another somewhat common problem, is the voltage switch on the power supply set to 240 Volts when you are using 120 Volt power.

Next, make sure EVERYTHING is seated properly, and that no cables are backwards.

You might want to check the drive power connectors to make sure a wire isn't swapped in one of them.

The next step is to start swapping components.
I would begin with the power supply.

Chas

This is a simple mistake, did you double check the connections for the case connections to the motherboard? Most are vary simple but there are a few if reversed or if put in the wrong plugin will cause problems. Otherwise thechas is right.

With ATX systems, there is often two power switches: One in the back and one in the front. Make sure the one in the back is turned on...

A device on your computer is not connected correctly.Remove all ribbon cables and power cables to your devices (hard drive;stiffy;cd-rom etc).Also remove all cards except the video card (if not onboard) and then try to start the system.If it works,then install one device/card at a time to complete the installation.

This may not apply to you but I had an interesting experience recently. I have a UPS on my workbench that I plug my systems into when building them (in case of power outage during a bios flash or something). I had a few systems that exhibited similar behavior to what you are describing. I found that my UPS battery was old and the line voltage would drop when the power switch on the PC was turned on. Some machines would come up fine but others would just reboot or the fan would rev up and down and some would just fail to start. Changed the UPS battery and problem went away.
Might not help you but I hope it helps somebody.

Try removing the PCI cards.
Generally it should work.
if not reseat the memory.

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