|Troubleshooting a non-booting system has been described here many times before.|
The fact your motherboard is getting some power, and fans spin, does not guarantee your powersupply is fully functional. Nor does it mean the PSU is the problem or the only possible problem.
Begin by disconnecting from mains power and re-seating all the power & data cables within the case. Remove the ram from it's slots and re-insert several times. Do the same with graphics card and any other PCI cards. Re-connect the power and try again. Has anything changed? This test at least eliminates a loose or corroded connection. Check the condition of the power connectors to the motherboard. I have had one where there was a loose connection on the 4 pin auxiliary CPU power and it had been arcing until the plastic partly melted. I replaced the powersupply. If you can, borrow a known working powersupply.
Next step is to reduce the system to barebones configuration. Disconnect all power & data cables from optic & hard drives. Remove all but 1 stick of ram. Remove any/all PCI cards. If the motherboard has an on-board graphics use that and remove any AGP or PCI graphics card.disconnect all external devices except monitor & keyboard. Leave only Powersupply, CPU with heatsink fan. 1 stick ram, Keyboard & graphics.
Now try again to boot up into BIOS. Any beeps? Remove the remaining ram and try booting again. Any beep?
If you cannot get any BIOS beeps with a known good PSU and the system in barebones then you have reduced the possibilities to Motherboard or CPU.
If it will boot up in barebones mode then you know it is one of the peripheral devices causing your problem. re-attach things one at a time until boot failure occurs. I have had systems fail to boot due to bad PCI cards/slots, bad optic drives, bad hard-drives, failed CPUs, bad ram, and the burned-out powersupply/connectors. You only determine what's wrong by testing with known good spare parts.
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