If moving your mouse or pressing the keyboard turns on your PC
Even if you shutdown your PC (not just sleep or hibernate) your mouse or keyboard can start it up. This is related with the options of your motherboard. For example Asrock motherboards are known to offer this option.
To solve the problem you need to enter to the configuration panel of your motherboard (press the delete button when the PC boots) and go to the ACPI (power management) settings. Try to locate there the relevant options, such as “USB Mouse Power On” and “USB Keyboard/Remote Power on”.
If you prefer to turn on your PC only by pressing the power button on the PC case, disable both of these options, preventing your mouse and keyboard to turn on the PC.