Step 1: Remove or disconnect all components from the system: Unplug all PCI cards except video ( modem, network, sound, etc), Unplug the cables that connect the floppy, hard drive(s) and cdrom(s) to the motherboard from the motherboard.
You want to have only the motherboard, processor, memory, keyboard, monitor, and video card hooked up. Unplug the mouse but not the keyboard. Boot up and enter the BIOS, reset to default all settings. Save and exit.
Boot up the system and report what you see if it does not POST properly. If the POST is successful, plug in the hard drive by itself and boot up again. If successful, plug in the CDROM drive and boot up. If successful, continue plugging in components ONE at a time, booting up between plug-ins until the system does not boot properly after you plug one in. The last component you plugged in when it does not boot up properly is the problem, and needs to be replaced. (it could be the cable that plugs in the component if it was the floppy drive or hard drive or cd-rom drive that made the system fail).
If in the beginning after you reset all BIOS settings to default it still fails, then the motherboard, processor, memory, or video card are going to be the problem. Check all jumper settings, if more than one memory chip then remove first one, then check boot, if not working replace the memory chip with the other one and test boot again.
Also if there is built in diagnostics onboard the BIOS, run the tests with everything but motherboard, processor, memory, video, keyboard, and monitor plugged in.
Report back to me once you do as instructed.