Welcome to BC!
We hear this complaint a lot around here! You are not alone.
The best way to track this down is to 'strip' the motherboard down to the essentials. In other words, disconnect hard drives, floppy drives, optical drives, etc, so that there is only motherboard, power supply, one piece
of RAM, CPU and CPU fan. Then try to boot. If you get the same result the next step is to swap the RAM pieces and try again. If problems persists: Remove motherboard from case, put it on a piece of cardboard or something to insulate it from shorting out on anything, and then try to boot again.
The most common reasons for this is connections not correct, and motherboard shorting out on case.