What are your power supply specs?? is it strong enough for an a64 system??
Try to remove all the hardware sept the CPU w/fan and heat sink, the power supply, 1 video card, and 1 stick of memory.
Remove the power, or turn the rocker switch off on the power supply then pull the battery out of the MB, reset the CMOS for an hour or more by moving the jumper on the MB. Replace the battery, reposition the CMOS jumper to the normal position, turn the rocker switch on the power supply back on, and try to boot again.