A few days ago, I noticed that one of my fans was making a loud noise (nothing too alarming, just louder than NORMAL). Not wanting it to run all night in case the fan decided to croak in the middle of the night, I shut my computer off. The next morning, my machine did not come on again. At all. No lights, no sounds, nothing. Because of this, I figured it was my power supply (which, I thought to myself, would explain the loud fan noises from the previous day.)

So, I beboped on down to the electronics store and, instead of buying a new power supply, one of the many cases on sale caught my eye. And it came with a power supply. Thinking this would solve all my problems, I came home and transferred all my hardware to the new tower. Eagerly expecting my computer to be back to its old self again (this time with a shiny new pink case - I'm a girl, I can have a pink case, darnit!), I PUNCH the power button and... well, nothing. No lights, no sound, nothing.

I guess it wasn't the power supply. Any ideas on what else it could be? Would a fried motherboard act like this, perhaps?That's like saying:  "My car wouldn't start, so there must be something wrong with the engine.  So I WENT and put a new engine in the car, and it still wouldn't start".  Before the replacement, you had 1 variable, now you have introduced many more variables.  It might have been the PSU & you might have messed up the build.  Diagnose the problem before throwing new parts at it.

If the fan making the loud noise was the CPU fan, the computer won't run if that fan is dead.  Maybe the fan is choked with dust & just won't spin which the motherboard will see as dead.

Loud fan does not indicate bad psu, but could be bad fan bearings.wow commando way to be a *censored*. how is changing power supplies adding a ton of variables.... if anything it takes out one.

you can test the power supply with a paper clip ( http://www.overclock.net/faqs/96712-how-jump-start-power-supply-psu.html )
i have had a similar issue like this and it turned out my motherboard was crap. unfortunately with a motherboard being bad one way i can think to test it would be getting a new mother board but that cost a lot of money. or if you have a PC store near by you can take it in and for around $40 they can tell you what is wrong. that i believe is the easiest way. when my motherboard was crap i just didn't bother trying to get a new one i just built PC. if you do get a new motherboard dont get a Intel DP55WB motherboard there crap.

hope this helps slightly.
What Commando is saying is right. If you start adding in componets to the machine, thinking it might solve the problem, well, you're like a dog chasing it's own tail.
 
I had a similar problem to this. I built a new machine and it would not turn on. So, I removed all the bits N peces including extra RAM and PCI cards etc. So I just had a simple, basic PC. Motherboard, bootable HDD, 1 Stick of RAM and PSU. Then it worked. I then plugged in the extra bits back in one at a time. First added ANOTHER RAM stick. All good, then I added Graphics card. And thats where the problem was. Nothing to do with the power supply.

So, start with a basic machine and then troubleshoot from there. Have a look at the motherboard (helps if you have a flash light) maybe a capacitor blew.

Good luck.