typedef enum _IO_PRIORITY_HINT
{
         IoPriorityVeryLow = 0,
         IoPriorityLow = 1,
         IoPriorityNormal = 2,
         IoPriorityHigh = 3,
         IoPriorityCritical = 4,
         MaxIoPriorityTypes = 5
} IO_PRIORITY_HINT;

Windows Vista Kernel Structures