курим шпаргалку от разработчика:
dwReason [in]

The reason for initiating the shutdown. This parameter must be one of the system shutdown reason codes.

If this parameter is zero, the SHTDN_REASON_FLAG_PLANNED reason code will not be set and therefore the default action is an undefined shutdown that is logged as "No title for this reason could be found". By default, it is also an unplanned shutdown. Depending on how the system is configured, an unplanned shutdown triggers the creation of a file that contains the system state information, which can delay shutdown. Therefore, do not use zero for this parameter.

подробнее здесь: http://msdn.microsoft.com/en-us/library/aa376868(VS.85).aspx
и здесь: http://msdn.microsoft.com/en-us/library/aa376885(v=VS.85).aspx