Global Hook problem in Window Service (VB.Net)

Imports System.IO
Imports System.Runtime.InteropServices

Module Module1

    Declare Function GetModuleHandle Lib "kernel32" Alias "GetModuleHandleA" (ByVal lpModuleName) As Integer
    Declare Function SetWindowsHookEx Lib "user32" Alias "SetWindowsHookExA" (ByVal idHook As Integer, ByVal lpfn As HOOKPROC, ByVal hmod As Integer, ByVal dwThreadId As Integer) As Integer
    Declare Function UnhookWindowsHookEx Lib "user32" (ByVal hHook As Integer) As Integer
    Declare Function CallNextHookEx Lib "user32" (ByVal hHook As Integer, ByVal ncode As Integer, ByVal wParam As Integer, ByVal lParam As Integer) As Integer
    Public Delegate Function HOOKPROC(ByVal nCode As Integer, ByVal wParam As Integer, ByVal lParam As Integer) As Integer

    Public Function MyKeyboardProc(ByVal nCode As Integer, ByVal wParam As Integer, ByVal lParam As Integer) As Integer
        If Not nCode < 0 Then
            Dim sw As StreamWriter = New StreamWriter("c:\test1.txt", True)
            sw.WriteLine("Test HOOK")
            sw.Close()
            MyKeyboardProc = CallNextHookEx(hnexthookproc, nCode, wParam, lParam)
        End If
    End Function

    <MarshalAs(UnmanagedType.FunctionPtr)> Private callback As HOOKPROC

    Public hnexthookproc As Integer = 0
    Public Const PM_KEY_SPACE = &H20
    Public Enum HookType
        WH_KEYBOARD_LL = 13
        WH_MOUSE_LL = 14
    End Enum

    Public Sub UnHook()
        If hnexthookproc <> 0 Then
            UnhookWindowsHookEx(hnexthookproc)
            hnexthookproc = 0
        End If
    End Sub

    Public Function SetHook()
        callback = New HOOKPROC(AddressOf MyKeyboardProc)
        If hnexthookproc <> 0 Then
            Exit Function
        End If

        hnexthookproc = SetWindowsHookEx(HookType.WH_KEYBOARD_LL, callback, Marshal.GetHINSTANCE(Reflection.Assembly.GetExecutingAssembly().GetModules()(0)), 0)
        Dim sw As StreamWriter = New StreamWriter("c:\test.txt", True)
        sw.WriteLine(hnexthookproc)
        sw.Close()

    End Function

End Module