From your diagnoses you may have a bad Power On switch, or a bad Power Supply.
An ATX power on switch is connected to the motherboard's Front Panel header.
From there the circuit is continued through the ATX main power cable, and into the Power Supply.
The Power On switch connects the Soft Power On wire, to Ground.
(The Soft Power On wire is abbreviated as PS_ON. The wire is Green)
If the Power On switch is bad the Power Supply will not turn on.
A bad Power Supply will have a weak voltage power rail, (Or more than one)
Enough power to light LED lights, and perhaps spin fans, but not enough power to turn the Processor on.
No Processor operating, no computer.
1) If all of the LED lights were on at once, they would use less than 1 Watt of power.
2) EACH fan uses 2 to 3 Watts.
3) A typical Processor can use from 51 to 125 Watts. Depends on what Processor it is.
(Older Intel Pentium III's, and comparative AMD processors used less than 51 Watts)
You may have enough power to light that LED light, but not enough to spin fans, or turn the Processor on.
There is a simple test to see if the problem is the Power On switch, or the Power Supply.
Bypass the Power On switch.
A jumper wire is used to temporarily connect the Soft Power On wire to ANY Ground wire.
The connection is made at the Back of the ATX main power cable's connector, with the ATX main power cable plugged into the motherboard, and computer plugged into power.
1) The Asus M2N-MX (And SE version) motherboard uses a 24-pin ATX main power cable.http://www.asus.com/Motherboards/AMD_AM2/M2NMX_SE_Plus/
This is one example of a 24-pin ATX main power cable, and it's respective connector on the motherboard,http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain24
Looking at the photo to the far right, shows the 24-pin ATX main power cable plugged into the motherboard.
This is the position for the test.
(Computer also plugged into power)
The BACK of the connector is where the wires go into the connector.
The jumper wire is used on the Back side of the connector.
One end of the jumper wire goes down into the socket hole with the Green wire.
(Soft Power On wire)
The other end of the jumper wire goes down into ANY socket hole with a Black wire in it.
ALL Black wires are Ground wires.
The jumper wire goes down into the socket hole RIGHT NEXT TO the existing wire.
The jumper wire MUST go down into the socket hole far enough to touch a metal terminal.
At the end of every wire going down into the ATX main power cable's connector, is a metal terminal.
The metal terminal is pretty far down in the socket hole.
The preferred jumper wire used is a paper clip.
The paper clip's diameter is small enough, to go right next to the existing wire in the socket hole, and touch the metal terminal.
The paper clip is straightened out, then bent into a U-shape.
The 'legs' of the U-shape go down into the two socket holes.
The middle is wrapped with a few wraps of black plastic electrical tape.
The taped area is for your finger, and thumb to hold onto.
The voltage is 5 Volts DC.
Two D cell flashlight batteries produce 3 Volts DC.
There may be a small spark when you make the connection, advising you in advance.
The connection made is no longer than 2 seconds.
(The Power On switch is a Momentary Contact Switch)
Computer comes on, (Power Supply actually) you know you have a bad Power On switch.
Computer does Not come on, you know you have a bad Power Supply.
For additional questions please post in a Comment.