You can set the power button to do "nothing", so if there's a short there, the server should keep up and running. You will have to shut down via software since that moment on, which is actually better, in my view, if your machine is supposed to stay up all times.

Also, shut down events must be triggered by something, whether that is a power button push or a software initiating the shut down process. That "something" ends up in the event log. You say that you did not find anything around the time of the shut down. Do you mean you did not find anything "unusual", or do you mean the shut down event itself was not logged? If the latter, that's not ok and makes me think of a non-graceful shutdown, regardless of XP asking you to boot in safe mode or not uopn restart.

Can you post here the events that preceed and that follow the down time? Maybe we can help with those.