How to stop Computer Starts Directly when Power On

I have faced this problem many times in My Networking Lab. When I start power supply to my PC it starts automatically without pressing power button on my cabinet. I want to stop Computer Starts directly when Power On. Although PC works fine but I want to stop direct power on. I changed BIOS Settings to Default. But the Problem Not Solved. What is the main reason behind this Problem and how to shootout this problem.
