[Apeside]

I'm 99% sure it's your PSU can borrow one from someone and test it on your pc ?
Before you got this problem your pc restarted from time to time ? if you noticed anything weird before getting here pls tell us it might help .

Before this problem I had to turn off the computer using the PSU switch.

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When I turn the PSU on the CPU fan works,

 

 

This should not happen until you press the power on button on the MB, the CPU is powered by a separate 4+4 pin 12V ATX supply from the PSU which should not activate until the MB sends a power good signal to the PSU, this could be caused by a bad MB and the only way to check is to swap in a known good PSU, please note that if this has been caused by a short circuit one or more items of hardware could have been fried.

[phillpower2]

When I turn the PSU on the CPU fan works,

 

 

This should not happen until you press the power on button on the MB, the CPU is powered by a separate 4+4 pin 12V ATX supply from the PSU which should not activate until the MB sends a power good signal to the PSU, this could be caused by a bad MB and the only way to check is to swap in a known good PSU, please note that if this has been caused by a short circuit one or more items of hardware could have been fried.

[Apeside]

ok. I'll see if I can get hold of a PSU and try again.

 

THANK YOU! For all the help, I really appreciate it!

[phillpower2]

One final check for you to do is to disconnect the PSU from the wall socket and then the CPUs 12V ATX supply and the MBs 24 pin main ATX supply from the MB, reconnect the PSU to the mains, switch the PSU to the on position and see if the PSU activates on its own.

 

You are most welcome btw 

[Apeside]

Best Answer

I think my computer is now ready for PC Heaven - the recycling bin.

 

Thanks again for the kind help!  

[phillpower2]

Did you try the suggestion in my reply #19, if not give it a try, if you have what happened.

 

I would not give up on anything until an alternative PSU has been tried.