First, make sure that you have any parameters in BIOS regarding rebooting on certain events, mouse, keyboard, NIC, etc., etc. disabled.
If that does not work, then you should look at your motherboard and make sure that you have the power connections that come from the power button on your case to your motherboard, are connected correctly onto the motherboard terminals/pins, i.e. the correct connector to the correct motherboard pins and also make sure that the polarity on each one is not reversed - please see your motherboard owners manual for the correct orientation of these connectors.
Good luck.