First thing to check with a desktop computer problem is the Power Supply.
This I state a lot.
In reality there's a flaw.........
In reality a person should visual check for bad capacitors on the motherboard, then move to the Power Supply.
The AsRock Conroe865GV uses nothing but Electrolytic Capacitors.
(Radial Aluminum Electrolytic Capacitors to be exact)
Doesn't use any Solid Polymer Capacitors.
These are known to be a 'weak link' on a motherboard.http://capacitorlab.com/visible-failures/index.htmhttp://www.youtube.com/watch?v=Lut7MX5Dd_A
Capacitors are used as Filters, and Voltage Regulators, on the motherboard.
The ones used as voltage regulators are in the Motherboard Voltage Regulator Circuit,http://www.hardwaresecrets.com/article/616
Just thought you might like to know
Since you indicate the CPU fan is spinning, I assume you know the computer isn't supposed to be dirty inside.
If you don't............we need to have a long talk..........
With a Power Supply having a weak voltage power rail, you may see LED's light, and maybe fan's spin, but there won't be enough power to turn the CPU (Processor) on.
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. Just depends on what CPU it is.
The motherboard chipset is an Intel 865GV chipset,http://en.wikipedia.org/wiki/List_of_intel_chipsets#Pentium_4_chipsets
Can support Intel Pentium 4 HT, models 620 through 672; for ->one.
They can use Up To 115 Watts,http://en.wikipedia.org/wiki/List_of_Intel_Pentium_4_microprocessors#Prescott_2M_.2890.C2.A0nm.29
[ AsRock Support states the Conroe865GV will support Intel Core 2 processors, but I don't believe it.
It isn't until you get to the Intel 945G, that Core2 support is picked up.
Just my 2 cents ]
High voltage AC electricity comes into the Power Supply, and is contained in it's metal case.
The AC voltage is converted into low DC Voltages.
3.3 Volts DC, 5 Volts DC, and 12 Volts DC.
In comparison two D cell flashlight batteries produce 3 Volts DC.
A multimeter is used to check the three voltages.
Need guidance with this post back in a Comment.
[An economical multimeter around here, can be purchased for around $5 to $12.
Available in a lot of stores. I have seen them on checkout aisle racks, of major discount stores ]
Use a KNOWN to be good, Compatible power supply for a test unit.
May be a working computer that is not being used, that you can Temporarily borrow it's power supply.
Let's start the diagnosis from here..........
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