As soon as I plug my PC in it turns on but nothing else happens. My fan and motherboard turn on but the computer does not BOOT up. I have swapped out a POWER supply which I know works, and the same things happens. Any help is much appreiciated.What happened prior to this, what are the specifics of the machine, etc., etc.

See below.Sorry, I guess more details would definitely be hopeful for you. What happened was, out of the blue the PC just turned off. Since then this business of turning on the second it gets power has been happening. Now I took it in for repair, and they told me it was my processor and motherboard. I REPLACED my them, the processor is now an AMD Ahtlon 3200 and the motherboard is a ASUS A8V-MX. However the same PROBLEM still persists. The power supply is 300 watt. I hope this is enough information, if not ask me what you would like to know. Thanks again for any help.300W is on the lower end for newer machines...i would consider replacing the PSU.
Don't buy a cheap one they are nothing but headaches.
The other possibility would be a problem with the actual power switch itself. Find out if the shop tested this with a meter.

patio. 8-)And the burning question is a 300 watt what? What brand is on it?The power supply brand is Enhance, and the model is ATX-1125bta.I normally won't spend less than $75.00 on a PSU but that's just me...

patio. 8-)Enermax make nice PSU's along with many other makes but avoid cheap rubbish and many that come with cases.

Are you sure its not the hard drive?, Does anything display on the monitor?, Any beeps?

Nothing on the monitor, no beeps, just the motherboard light, and the cpu fan. This occurs even when the hard drive isnt connected.Quote

Sorry, I guess more details would definitely be hopeful for you. What happened was, out of the blue the PC just turned off. Since then this business of turning on the second it gets power has been happening. Now I took it in for repair, and they told me it was my processor and motherboard. I replaced my them, the processor is now an AMD Ahtlon 3200 and the motherboard is a ASUS A8V-MX. However the same problem still persists. The power supply is 300 watt. I hope this is enough information, if not ask me what you would like to know. Thanks again for any help.