I insatlled a used socket 7 motherboard and AMD K6-2 380mhz CPU in my computer, all other parts were used by a Slot 1 PC before. when I switch on the machine, the power supplier and cooler's fan is runing, but nothing shown in screen and no any bip sound. I changed all parts--CPU, Power supplier, memory, VGA card, system speed jampers, hard disk, but it still no boot. Someone said to me, this is mean that there are problems in the memory or VGA card, but I changed them already, it still not working. Someone can help me? Thanks.

Disconnect all the drives and remove all the cards except the vga card. That will isolate the problem to the CPU or motherboard.

Make sure your speaker is plugged into your motherboard, different beeps can tell us what the problems may be... As mentioned before you should remove all IDE cables and all devices other than graphics, ram and CPU.. Try one IDE device like a HDD on its own as some older motherboards do not kick in unless such a device is present (very rare, QDI usually) If possible you may want to try those parts in another PC to see if they work ok... especially the CPU.. That way you can narrow everything down to the motherboard. Make sure your power switch and LED's are all the right way as if they are not they can act as resistors... also make sure you motherboard is not earthing itself through your case (bad placement of spacers) Try all ram sockets. Good luck...