The computer can be completely hosed and still turn the fan on when you press the button (newer systems can have a southbridge blow and still power up but its dead).



Get yourself a $5 shipped ISA/PCI POST card from ebay, then you can plug it into a PCI slot and see what activity is going on and where the system hangs up. It also shows what voltages are present and if the reset line works. Also invest in a $5 power supply checker so you can verify the power works before using it in a system.