NO POWER
If you have changed the power unit and the computer is still not powering up, then it is quite likely there is a motherboard fault - probably in the power management circuitry, which expects the power_good signal from the power unit at a precise moment in the boot up process. A fault in the power management circuitry would prevent the computer from beginning its boot-up process.
Do the fans in the power unit come on when the computer is switched on? If they don't - this suggests a power unit problem. If the power unit fans do start working but the computer still doesn't boot, this suggests either a faulty or 'dry' connection (easy checked) or a motherboard fault.
If your computer 'works' but nothing is displayed, try pressing Control+Alt+Delete. If it then boots up it suggests a power unit problem. If not .. we come back to the motherboard again.
A low wattage power unit can cause the problem you describe. Every 128 Mb of Ram needs 4 watts of power. Add more for the motherboard (up to 30 watts..) then the floppy/hard drive/dvd etc .. If the power unit can't supply the necessary wattage, it won't boot. The older 250-300 watt power units aren't suitable for modern computers with XP/Vista. Ideally, you should be looking at 400 watts plus -it will tell you on the power unit.
Though to be honest, it does sound like a motherboard fault.
The only other thing that can cause a computer not to boot up (though it will begin the boot up process) are other faults ... When you switch on does it actually display anything on the screen .. and are there any 'beeps'? 1 beep indicates everything is ok .. 3 short beeps and 1 long beep indicate a memory problem (and the computer won't boot..). 8 beeps indicate a video problem (and nothing will be dislayed on your screen). Same with 9 beeps .. which indicates a bios fault - and the computer won't continue the boot process.
If you have nothing .. no power whatsover ... it comes back to the power unit/mainboard. If you're happy that the new power unit you have put in is ok ... suspect the motherboard. If you have access to any unwanted or 2nd hand motherboard ... connect that. If your computer boots up ... its the original motherboard at fault.
Alternatively, rather than replace bits and pieces at random to cure the fault, take your computer along to a computer shop. It will take them minutes to test it for you and identify the fault.
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