Power Supply failure. Weak Voltage power rail.
Blue light in the front is the Power On LED light. Green light in the back is on the Power Supply.
When a Power Supply is plugged into power, there is a 5 Volt Standby power present in the Power Supply.
When you press the Power On button, this in turn presses against a Power On switch.
The Power On switch is located inside the plastic Power On button.
This is a basic example of a Power On switch,http://www.directron.com/atxswitch.html
Pressing the Power On switch closes a circuit for the 5 Volt Standby power.
The 5 Volt Standby power is then directed to a circuit inside the Power Supply.
This action turns the Power Supply on.
Power then goes from the Power Supply to the motherboard.
The first chip to receive power is the BIOS chip.
[Chip and Chipset are slang terms for I.C.
BIOS = Basic Input/Output System ]
The BIOS chip has the BIOS program burned into it.
BIOS, (The BIOS program), looks to see what devices are installed, does a Ram Memory count, TURNS the Processor on, and hands the computer over to the operating system.
(Windows XP is one example of an O/S. Operating System)
1) ALL of the LED lights, use less than 1 Watt of power from the Power Supply.
[ The HP Pavilion a1250n desktop computer, comes with an ATX style of Power Supply, which has a reported Maximum Wattage rating of 300 Watts]
2) EACH fan uses 2 to 3 Watts
3) A typical Processor uses 51 to 125 Watts. Depends on what Processor it is.
[The HP Pavilion a1250n comes an AMD Athlon 64 X2 3800+
This particular Processor uses (Fits into), a Socket 939 processor socket.
The Athlon 64 X2 3800+ can use up to 89 Watts.http://en.wikipedia.org/wiki/List_of_AMD_Athlon_64_microprocessors#Athlon_64_X2
Look under TDP.
This is what a Socket 939 processor socket looks like,http://en.wikipedia.org/wiki/Socket_939
You have enough power to light those simpy LED lights, and spin the fans a few times, but not enough power to Turn the Processor on, and keep it on.
No Processor running, No computer.
What causes Power Supply failure?
1) Computer is dirty inside, as well as inside the Power Supply.
In reference to the Power Supply;
The Power Supply used in your computer is an SMPS.
Switched-Mode Power Supply.http://en.wikipedia.org/wiki/Smps
There are two cooling components,
A) The Fan
B) Heatsink's which are used internally. (Inside)
[Typical construction of a Heatsink is a plate of metal with tall, thin fins protruding from it.
The plate of metal absorbs heat, from whatever object it is placed against.
The tall, thin fins absorb the heat from the plate of metal, and radiate the heat away.
If a fan is used in conjunction with a Heatsink, the air flow from the fan goes in-between the fins, and around the fins, and help to dissipate, (Carry away), the heat ]
When the Fan's blades, center hub, and surrounding cage (Shroud) are clogged with 'Gunk', the Power Supply heats up.
Heat = Wasted Energy
The more heat, the more the Power Supply strains to keep up with the call for power.
Eventually components inside the Power Supply fail, and the Power Supply fails.
2) Low quality Power Supply.
Low quality components are used in the Power Supply
The ATX style of Power Supply is used in a Large percentage of computers out there.
Very readily available.
I would suggest you install a Power Supply with more Wattage.
A computer only uses the power it needs, and NO more.
Won't hurt your computer.
Will help your computer as there will be less strain on the Power Supply.
Also the 300 Watt rating on that Power Supply is bunk.
Padded rating to make the Power Supply look better than it is.
The actual Wattage rating, is more like 60 to 70 percent of what is stated.
(180 Watts to 210 Watts)
Need help in opening your computer case, and replacing the Power Supply, and/or a recommendation for a Power Supply, let me know in a Comment.
[BE SURE to follow Anti-Static Precautions, BEFORE you reach inside your unplugged from power, computer ]