If the replacement is a quality make and model and the original was a
cheap "no name" - then the money was well-spent.
However, there is a mobo setting which determines how the machine
responds to a power fail. I suspect that yours has been set (changed?)
to reboot automatically on power restore. Hence it will boot whenever
power is applied. The solution is to change the setting.
I can't recall the exact details (like pinouts) right now but the ATX
power connector carries +5V, maintained 5V and a "power up" line. The
maintained 5V is used on the mother board to power circuitry which can
switch the power supply on (not just the power switch but also LAN
etc). IIRC it signals to the PSU by connecting the "power up" (or
whatever it's formally called) line to 0V. It could easily be that
the line is shorted to a 0V pin.
The ATX pinout is available on line. Google.
Alternatively there are a few BIOS setting which alter power up
behaviour. Have you been playing with them?
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