I'm not sure if pressing the power button for 30 seconds is the documented way to "discharge the mobo" but normally these things have batteries. My experience with an MSI board was to take the battery out, AND short the
bios clr (clear) jumper for 10 minutes. Doing it for 5 minutes did not do the trick, it had to be longer for some reason. Note that normally just shorting bios clear will clear the bios, but this board had some manufacturing issues and needed to have the battery removed and the jumper shorted for a while.
Now, if that doesn't help, and I'm betting it won't, unplug everything but the power from the machine, that means the drives and memory too. If you see an onboard piezo buzzer, it should beep at this point when turned on. Plug things in one at a time, starting with the memory (it's okay to put all the memory in at once) and ending with the keyboard. It should beep for a missing keyboard each time.
* 7 years later, someone misread this... UPDATED *
"Unplug everything but the power" is something you do as you would with any change to your computer's components. You unplug the mains power, wait for the PSU to to discharge (your motherboard may have a light for this), ground yourself, modify the components by unplugging everything attached to the motherboard except the power connector, then proceed to test this and each change in isolation by listening for the POST beeps.
"Plug things in one at a time" does not mean "Hot plug things". Power down after each step, disconnect the mains power, wait for the discharge, ground yourself, add a component back. The step at which your board does not beep, is the step at which you've added a defective component or found a part of the motherboard that doesn't work as intended.