It's a dead PSU. Just because you get standby power lights illuminated on the motherboard does NOT mean that the power supply is still good.
I can't count the number of PSUs I've replaced in the past year, and at least half the time the standby power light is still on, but darned if the computer won't boot until I replace the PSU. At which point it fires right up.
So my computer was working fine yesterday, I got up this morning to find that it wouldn't boot up.
It cannot be a problem with the PSU as the motherboard lights are on. I've checked all the connections to make sure nothing is loose, everything is fine. I have also tried unplugging everything but still nothing. I've even tried discharging the CMOS data by changing the CCMOS jumper and removing/replacing the button battery and ATX power connectors. Nothing seems to work.
Any ideas would be greatly appreciated.