Computer turns back on after suspend or power off

Desktop computer always turns back on about two seconds after suspend or power off. It does this always with Mageia, Q4OS and Zorin OS, but it will stay off if entering standby or powering off from Windows XP. The motherboard is Supermicro P4SPA with Award BIOS. How can I probe the software and hardware to find out what's causing it and can I change some setting somewhere to make the computer stay off?