Dell Dimension 3000 wont start properly

[alex-JL]

I have a dell dimension 3000 that wont start properly.

When i press the button the light LEDs come on, the fan come on, all for a split second.
Then everything shuts off.

I checked the switch and made sure it is OK.

I made sure the connections from power supply to motherboard is firmly seated.
There is a green LED on the motherboard that is lit when the power cable is connected to system and plugged into outlet.

I also plugged the power cable into another working receptacle, same thing still occurs.

I disconnected power from all of the drives, then tried again, same thing occurs.

The only awkward thing is that when I remove the P2(4 pin) power connector from the motherboard and try again, then the system starts up but there is no video, not booting activity.

I put the P2 connector back on the motherboard and try to start but the same thing happens, the LEDs, the fan, etc come on all for a split second . . . then everything shuts off.

Any idea as to what is happening?
Appreciate any help.