Suggest see if the problem is the Power On switch, or Power Supply.
It's a simple test to perform. Involves bypassing the Power On switch, BUT does NOT involve the Power On switch itself.
There is no cutting of wires, nor the insulation of the wire.
Power Supply comes on you know you have a bad Power On switch.
Power Supply does NOT come on, you know you have a bad Power Supply.
Post back in a Comment.
In the meantime clean out the inside of the computer, if it's dirty; (Unplugged from power, FOLLOW Anti-Static Precautions), plus remove the Ram Memory, and clean the gold plated contact pins with a pencil eraser. Use air to remove the eraser dust, reinstall.
It could also be a hardware problem.
There could be a power supply, BIOS or memory problem.
Here are some easy checks for some of them that have served me well.
Check for a bad memory module.
Unplug and open the desktop. Remove all your Ram memory.
Clean out any dust bunnies you see.
Reinstall one memory module at a time, turning on the desktop, allowing it to boot up each time.
Remember to turn off the desktop and unplug it each time you remove or install a module, or anything inside your desktop.
Also, check the CMOS battry. It is a CR2032/CR2025 disk type lithium battery. It maintains the internal clock and the BIOS. Failure can cause boot problems, and unexplained or random shutdowns.
It should be replaced if its voltage is 2.7v or below.
whenever you open any computer to do any work, it should ALWAYS be turned off and you should wear an earthing wrist band (connected to an earth point) before sticking your fingers inside so you don't inadvertently blow anything up with the static electricity in your body.
The fan moving only slightly How did you determine that that the PSU is ok can you you try another one
If you changed motherboards and the problem stayed thats eliminated the motherboard as the culprit
The bios might need updating ?
Might be a virus or malware ?
Does it start up in safe mode ?
Click start control panel administrative tools computer management then select event viewer this may vary depending on what operating system you have extend the events there you will see errors in red you will no what exactly what the error was and at what time it occured or click start type in event viewer press ENTER you might be able work your way through the problem. Hope this helps.
The power saving mode is engaged when no signal comes from the pc - the pc is either stopped (start it) or it's defective.
If you can't start the pc then you will have to dismantle it and test the components, starting with the video card. If you don't know how to do that then take it to a repair shop - don't make the problem bigger by messing around with it.
Power supplies go bad over time, but it's also possible some card fell out of place inside there. Open it up, use the brush attachment on the vacuum to dust it out as necessary, make sure the cards, memory and heatsinks are seated securely, and try again. It's easy to buy a new power supply at NewEgg or locally; you want around 300-450W unless you bought dual videocards (then up to 650W.)
Since you upgraded the operating system ti windows 7. You need to to reinstall the driver of your video card. Driver is the video card is not installed properly that is why you have expanded display. Make sure that the video card driver is compatible with windows 7.
click on start then control panel then select device manager then let it load the select the little plus (+) sign to the left of dvd cd rom drives it should drop down another list of your dvd drive ...right click on that writing and select uninstall let it go through what it does then restart the pc and when it starts it should come up on bottom right when it boots that it found new hardware and it will install the drive again
Are speakers turned on with volume up?
Are there batteries in the speakers of not powered by the outlet?
Are the speakers plugged into the wall as well as the computer's earphone jack?
Is windows set on mute?