Not trying to be trite sir, but I think that is a broad unsupported statement........
If we look at the known universe...........
!O_O! What a minute?...........What?............Oh........
...fans, video card, cpu, ram, cd drive, hard drive, and power supply work; (Semi-Colon), as well as the motherboard itself...........
With all due respect what do you base all that diatribe on?
Fans spin, and LED's light up; and you're going to assume the other hardware components, must be working as well?
THAT, is your basis for your hypothesis?
A) You can see the fans spin......
B) You might hear the harddrive spinning it's platters, while it is in an endless loop............
C) You can press the optical drive's (CD/DVD drive) tray release button, and open the tray.......(To assume it's getting power)
But tell me how you assume the cpu, motherboard, ram memory, graphics (video) card, and Power Supply; to be good?
What do you base this assumption on?
With all due respect..............nothing.
Your observations are baseless.
Seem like I'm trying to be rude?
No sir. Trying to get you to think outside the box
What is the common 'denominator' here?
What hardware component is common to all the others?
That........if not working at full capacity, would not allow the others to work?
The motherboard is the 'Building Block' of a computer.
The CPU is the 'Brain'
The Power Supply is the 'Heart'.
No 'heart' you have nothing.
When diagnosing desktop computer failure, the first hardware component to diagnose; is the Power Supply.
Without it operating at full capacity nothing else works.
Past this point the diagnosis can go on. (Power Supply deemed to be good)
If not you will be pulling your hair out searching for answers.
1) If ALL of the LED's were on at once, they would use less than 1 Watt of power.
2) EACH fan uses 2 to 3 Watts of power.
3) A typical CPU (Processor) can use 51 to 130 Watts of power.
A Power Supply with a weak voltage power rail, will have power to light those simpy LED's, and maybe spin fans; but will not have enough power to turn the CPU -> ON.
Have an economical multimeter?
They range in cost from $5 to $12, usually.
I have seen them on checkout aisle racks, at major discount stores.
Auto parts stores have them, but usually more expensive.
With it you can test the 3 main DC Voltages, coming out of the Power Supply;
Again, all are DC Voltage.
In comparison two D cell flashlight batteries produce 3 Volts DC.
(The dangerous AC voltage is kept contained, in the metal case of the Power Supply)
Use a KNOWN to be good, Compatible power supply for a test unit.
("I need to borrow the Power Supply out of your computer honey.
No, no.......I'll put it back. This shouldn't take long.
What? Touch it, and I'll lose sum fingers?" )
Also, follow Anti-Static Procedures, and look at the Electrolytic Capacitors on the motherboard.
In fact, what the hey right? Might as well look at them first,
Capacitors on the motherboard are used as Filters, or Voltage Regulators.
The ones used as voltage regulators are in the
Motherboard Voltage Regulator Circuit.
Part of what the motherboard voltage regulator circuit does, is regulate voltage for the processor.
The processor MUST have a steady, 'clean', supply of voltage; and it MUST be kept within the tight voltage tolerance range, for the processor.
Too little, or too much, and it turns off. (BIOS turns it off)http://www.hardwaresecrets.com/article/616
(For older motherboards, and to include older server computer motherboards; voltage regulation for the processor was accomplished with a VRM. Voltage Regulator Module.
A separate, replaceable 'card' )http://www.msi.com/product/mb/MS-6378.htmlhttp://computer.howstuffworks.com/hard-disk.htmhttp://en.wikipedia.org/wiki/Computer_power_supply#Wiring_diagramshttp://www.playtool.com/pages/psuconnectors/connectors.html#atxmain20
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(One last little item that seems to slip through the cracks, the Ram Memory. Clean the gold plated contact pins with a pencil eraser. Use air to remove the eraser dust )