Let's diagnose from your statements;
A) No display on monitor
B) Harddrive activity LED is on
C) Fan keeps running. Assume your reference is to the computer case fan. IMHO if it was a reference to the Processor fan, you would have stated so.
D) Power Supply changed. Assumption is that it is KNOWN to be good......
A1) Following a diagnostic flowchart, you would rule out the possibilities of a problem, one by one, and step by step.
No display on the monitor can mean a lot of things, however with the additional information we can narrow it down further....
B1) The Harddrive activity LED is on. The way you made the statement, makes me think you are referring to it is CONSTANTLY on.
Harddrive is in an endless loop. BIOS is trying to find the Boot record on the Harddrive.
However for some reason it isn't finding it.
C1) Fan keeps running....
This leads me more to believe the reference is now, to the Processor Fan.
Processor fan running at high speed.
D1) When someone states they 'Changed' the Power Supply, instead of Replaced the Power Supply, this leads me to believe the reference is to another Power Supply that was laying around, and not a new Power Supply.
Therefore makes me want to ask;
"How do you know the Power Supply is indeed good?
All 3 main voltage power rails are good?
3.3 Volt, 5 Volt, and 12 Volt? (DC)"
A Power Supply with a weak voltage power rail, will show the symptoms you are seeing.
1) If ALL of the LED's (Light Emitting Diode) 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 Processor can use 51 to 130 Watts of power.
Just depends on what Processor it is.
No Processor running, no 'Brain'
No 'Brain' running, no computer.
You press the Power On button. This plastic assembly then presses against a Power On switch, located inside it.
The Soft Power On circuit is activated. This 'excites' the Power Supply, and turns it on.
The first chipset to receive power is the BIOS chipset.
The Basic Input/Output System program is initialized.
BIOS looks to see what devices are connected, does a Ram Memory count, TURNS THE PROCESSOR ON, and hands the computer over to the Operating System.
(In this case the O/S is some version of Windows)
Processor is NOT turning on. No 'brain' operating to find the Boot record on the Harddrive.
Harddrive spins in an endless loop.
Power Supply is KNOWN to be good? Came out of a working computer?
Power Supply is a new one?
Still doesn't mean it is good. Test the 3 main voltage power rails, with a multimeter.
Assuming the Power Supply IS indeed good......
Clean out the inside of the computer.
Replace the Thermal Paste on the Processor (CPU)
Make SURE you are following Anti-Static Procedures.
[The CPU (Processor) is the MOST susceptible hardware component to Static shock ]
However, after replacing the Thermal Paste, do not plug the Power Supply into power.
Press the Power On button in, and hold in for a count of 10 seconds.
Do this procedure 2 more times.
What you are doing is clearing CMOS Error Codes, and resetting BIOS Setup back to the factory default settings.
The former matters, not the latter. Just telling you the 'whole story'.
NOW plug the Power Supply into power. WAIT 1 minute, turn the computer on.
Post back in a Comment with the results. Or click on Reply at end of solution, and post results.
Need guidance in replacing the Thermal Paste on the Processor (CPU)?
Post the computer manufacturer name, and model number.
The Model Number can be found on the Back of the computer, or up on the side of the computer tower.http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=3071621&CatId=503http://www.hardwaresecrets.com/article/274