|Strip it down to barebones. ie: leave only motherboard, powersupply, CPU with HSF, 1 stick ram, graphics (onboard if available but plug-in card if needed), and keyboard.|
disconnect everything else including harddrives, floppy drive, optic drives and any extra pci cards etc.
If it still will not POST then you have isolated the problem to one of these components. Even without a stick of ram or graphics card you should still get error beeps as long as PSU,CPU and Motherboard are functioning.
If it does boot in barebones mode then add back one extra component at a time (powering off between each) until you get a failure.
I have seen boot failures caused by faulty harddrives, optic drives, Network cards other failed PCI devices, as well as failed motherboards, CPUs and blown powersupplies.
Identifying what your particular problem might be is a process of elimination.
I used to have a signature but it disappeared and I just couldn't be bothered writing another so please feel free to ingore this.