|"My computer's power plug had been supplied with a 13amp fuse (SHOULD BE 5AMP, EVERYBODY) so it's a miracle it lasted as long as it did."|
In North America (I'm in Canada), computers are not required to and don't have a fuse in the power cord.
Computer power supplies are not 100% efficient. E.g. a quality one may be 85% efficient, a cheaper one much less efficient. In any case, the fuse in your power cord needs to be at least the amperage rating stated on the power supply's label that it draws for the AC voltage you are using. The AC wattage (amps X voltage) is always more than the max total wattage for the DC voltages of the power supply's stated capacity.
It doesn't matter much if the fuse rating in your cord is more than that. The reason for a PS failing has nothing to do with that, and if the PS shorts, if that gets to the AC input it will blow any fuse in the cord, and here, where no fuse is required, it would probably trip the circuit breaker for the circuit the computer is connected to.
There is a fuse inside all computer PSs I've seen the insides of, but usually it does not blow when the PS fails.
Since both your PS and mboard were replaced, it is quite possible you did connect firewire port or USB port wiring to the wrong header.
ATX mboards are always powered in some places even when the computer is not running as long as live AC is being supplied to the ATX PS, the PS is switched on, and at least the main power wiring connector from the PS is connected to the mboard.
You MUST unplug the computer, or switch off the AC source it connects to, whenever you make any changes to connections or plug in or unplug any component inside the computer case that connects to the mboard or the drives, otherwise you risk damaging something.
Did you always do that?
"They should ALL work for USB 1.1 in Windows in any case if all the USB controllers are enabled in the bios.
E.g. a USB connected keyboard or mouse should work in ALL USB ports directly connected to the mboard.