Shutting down Windows XP - AdjustTokenPrivileges failing with error code 1300

Const SE_PRIVILEGE_ENABLED As Integer = &H2
Const TOKEN_QUERY As Integer = &H8
Const TOKEN_ADJUST_PRIVILEGES As Integer = &H20
Const SE_SHUTDOWN_NAME As String = "SeShutdownPrivilege"
Const EWX_LOGOFF As Integer = &H0
Const EWX_SHUTDOWN As Integer = &H1
Const EWX_REBOOT As Integer = &H2
Const EWX_FORCE As Integer = &H4
Const EWX_POWEROFF As Integer = &H8
Const EWX_FORCEIFHUNG As Integer = &H10
 
Friend Structure Luid
    Public Count As Integer
    Public Luid As Long
    Public Attr As Integer
End Structure
 
Private Declare Function GetCurrentProcess Lib "kernel32.dll" () As IntPtr
 
Private Declare Function OpenProcessToken Lib "advapi32.dll" (ByVal h As IntPtr, ByVal acc As Integer, ByRef phtok As IntPtr) As Boolean
 
Private Declare Function LookupPrivilegeValue Lib "advapi32.dll" Alias "LookupPrivilegeValueA" (ByVal host As String, ByVal name As String, ByRef pluid As Long) As Boolean
 
Private Declare Function AdjustTokenPrivileges Lib "advapi32.dll" (ByVal htok As IntPtr, ByVal disall As Boolean, ByRef newst As Luid, ByVal len As Integer, ByVal prev As IntPtr, ByVal relen As IntPtr) As Boolean
 
Private Declare Function ExitWindowsEx Lib "user32.dll" (ByVal flg As Integer, ByVal rea As Integer) As Boolean
 
Private Sub DoExitWindows(ByVal flg As Integer)
    Dim tp As Luid
    Dim hproc As IntPtr = GetCurrentProcess()
    Dim htok As IntPtr = IntPtr.Zero
 
    OpenProcessToken(hproc, TOKEN_ADJUST_PRIVILEGES Or TOKEN_QUERY, htok)
    tp.Count = 1
    tp.Luid = 0
    tp.Attr = SE_PRIVILEGE_ENABLED
 
    LookupPrivilegeValue(Nothing, SE_SHUTDOWN_NAME, tp.Luid)
 
    AdjustTokenPrivileges(htok, False, tp, 0, IntPtr.Zero, IntPtr.Zero)
 
    ExitWindowsEx(flg, 0)
 
End Sub