Test the Power Supply first.
If you have a multimeter, use the Positive (Red) probe lead to connect to the power wire to be checked.
The Negative (Black) lead connects to ANY ground wire. (ALL Black wires are Ground wires)
Three main voltages are produced by a personal computer Power Supply.
1) Orange insulated wires are 3.3 Volt
2) Red wires are 5 Volt
3) Yellow wires are 12 Volt.
(Again, ANY Black wire is a Ground wire)
Or use an economical power supply tester.
You are making a connection from the green Soft Power On wire, to any Ground (Black) wire with that jumper wire.
Using an HP Pavilion A1330E desktop computer for an example,
we can see that the main power cable is a 24-pin ATX main power cable.
This is readily shown by looking at the motherboard,http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00496280&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&lang=en&product=1829044
Scroll down to the motherboard illustration, and the motherboard photo underneath the illustration.
This is the pin-out for a 24-pin ATX main power cable, and connector,http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain24
The Green wire (Pin 16) is the Soft Power On wire. The other end of the jumper can touch ANY Black ground wire.
1) ALL the lights use less than 1 Watt of power.
2) EACH fan uses 2 to 3 Watts.
3) A typical Processor can use anywhere from 51 to 125 Watts. Depends on what Processor it is.
The three Processor options for the Pavilion A1330E are, (Socket 939 style),
A) AMD Athlon 64 X2 4800+
B) AMD Athlon 64 4000+
C) AMD Sempron 3200+
The Sempron 3200+ can use up to 62 Watts
Athlon 64 X2 4800+, 110 watts, and Athlon 64 4000+ can use up to 89 Watts.
You have nothing but the motherboard plugged in, and the Power Supply takes a 'nose dive'.
Power LED turns off.
Bad Electrolytic Capacitors in the Power Supply.
Replace power supply.
What causes this?
1) Most computer failure can be attributed to the inside of the computer is dirty, as well as the Power Supply.
When the cooling components of the Power Supply, (Fan, and internal Heatsinks), are clogged, the cooling capacity drops tremendously.
Heat = Wasted Energy
The Power Supply tries to keep up with the call for power, and eventually components inside the Power Supply fail.
A can of compressed air for computers, used on a regular basis as needed could prevent this. (Observe Anti-Static precautions Before placing hands inside computer)
2) Cheap quality power supply.
Saves the computer manufacturer money.