Hi minter. You need to get back to basics.
First, if you can beg, borrow or otherwise obtain a digital voltmeter, you can check that the power supply is working by measuring the volts on a spare drive connector.
Then disconnect or remove all the drives and cards from the motherboard, leaving only the processor and minimum memory. If you can then get a POST beep, you will know that the motherboard, memory and processor are working. If you suspect the processor may be damaged, then if it's an Athlon, you can get a Duron to fit for about £20 and this will prove the other bits.
The power supply gets its on-off command via the motherboard ATX connector. Have you double and triple checked the switch connector and the ATX power connector? Have you also checked that none of the motherboard tracks have shorted to the case? You can confirm this when all the drives and cards are removed by unscrewing the motherboard from the case and standing it on a sheet of cardboard.