typedef enum _SYSTEM_POWER_STATE
{
         PowerSystemUnspecified = 0,
         PowerSystemWorking = 1,
         PowerSystemSleeping1 = 2,
         PowerSystemSleeping2 = 3,
         PowerSystemSleeping3 = 4,
         PowerSystemHibernate = 5,
         PowerSystemShutdown = 6,
         PowerSystemMaximum = 7
} SYSTEM_POWER_STATE;

Windows Vista Kernel Structures