The first thing you should do is remove the system from the case, place it on a non-conductive surface, and disconnect all components from the motherboard with these exceptions
- CPU (and heatsink/fan)
- A single stick of memory
- Video
- Power supply
- Power button
This means no drives, no peripherals, no extra ports, nothing. This tests two problems at once. They are the possibility of some peripheral preventing the system from powering up and the possibility of the motherboard shorting onto the case somehow (aka, a standoff that should not be there).
To do a quick elimination (only if the system is completely failing to give any power at all), find where the power button connects and short those two pins for a moment with anything conductive that you have on hand. A screwdriver, knife, coin, or anything metal will work. If the system spins up, you need a new power button. (If your system was already spinning up, you can skip this step.)
If the system fails to power up outside the case, here are two things you need to do. First, do a visual inspection of all the capacitors on the motherboard. These are the little battery-looking things. What you’ll be looking for is any fluid leaking out of the top or bottom, any “gook” anywhere on them, or if they are bulging out the top or sides.
While you’re poking around for bad capacitors, take a look at the ATX power connector; make sure it doesn’t have any scorch marks or look melted. If anything shows any of these characteristics, your motherboard is almost definitely your problem. If they all look ok, test repeatedly, with each stick of memory individually in each slot (this means nine tests for three sticks of memory on a board with three slots!).
If the system eventually powers up, you’ve found good memory — probably your only problem — and you’re probably done. Put the stripped-down version back into the case and secure it. If it powers up again, skip to step 9.
If not, you need to remove it again and investigate the setup of your motherboard standoffs and make sure nothing is touching the motherboard where it should not be touched. After this is done and all is well with the basics inside the case, skip to step 9.
If it fails to power up with any memory configuration out of the case, we know it must be either the CPU, the memory, the video, the motherboard, or the power supply, or any combination thereof.
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