Delll Dimension 5150 Won't Power Up?

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Delll Dimension 5150 Won't Power Up?

Dell Dimension 5150 won't power up. You try to power it on and 5 seconds later is shuts down. The light sequence is 1,2,3.. which means other, so I don't know what to take from that.
We have replaced the battery, power supply and motherboard. It still won't power up.

Right now, everything is unplugged from the power supply except the motherboard and nothing. Even took out the harddrive and tried to use a different one and nothing.

The only other thing I have been ready searching Google, is the usbs may be the issue... but I have also read that on this model it is very hard to get just the usb ports disconnected.