Public Class frm1

  Dim lngIconTray As IntPtr

 Private Sub frm1_Load(sender As Object, e As EventArgs) Handles Me.Load
    lngIconTray= My.Resources.MYICON.Handle
...
'Из кода (в процессе работы приложения -возможно через какое-то длительное время):
    Dim nid As New NOTIFYICONDATA
    With nid
      .hIcon = lngIconTray
...
  Shell_NotifyIcon(...

  Private Sub frm1_FormClosed(sender As Object, e As FormClosedEventArgs) Handles Me.FormClosed
  DestroyIcon (lngIconTray)

    Dim nid As New NOTIFYICONDATA
    With nid
      .hIcon = My.Resources.MYICON.Handle

Public Class frm1

  Public lngIconTray As IntPtr
  Public lngIconTray_Start As IntPtr
  Public lngIconTray_Stop As IntPtr

 Private Sub frm1_Load(sender As Object, e As EventArgs) Handles Me.Load
     lngIconTray_Start = My.Resources.ICONSTART.Handle
    lngIconTray_Stop = My.Resources.ICONSTOP.Handle
    lngIconTray = lngIconTray_Stop

  Private Sub ChangeTrayIcon()
    'код вывода иконки в трей
    Dim nid As NOTIFYICONDATA
    With nid
      .cbSize = IIf(sys_WinVista, Marshal.SizeOf(nid), Marshal.SizeOf(nid) - IntPtr.Size)' Len(nid)/NOTIFYICONDATA_V3_SIZE
      .hWnd = Me.Handle
      .uID = vbNull
      .uFlags = NIF_ICON
      .hIcon = lngIconTray
    End With
    Shell_NotifyIcon(NIM_MODIFY, nid) 'меняем иконку в трее
  End Sub

  'test (1000мс-меняем)
   Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
    If (lngIconTray = lngIconTray_Stop) Then
      lngIconTray = lngIconTray_Start
    Else
      lngIconTray = lngIconTray_Stop
    End If
    ChangeTrayIcon()
  End Sub

Public Class frm1

  Dim IconTray_Start As Icon
  Dim IconTray_Stop As Icon
  Dim bTest As Boolean

  Private Sub frm1_Load(sender As Object, e As EventArgs) Handles Me.Load
    '----------------------------------------
    IconTray_Start = My.Resources.ICONSTART
    IconTray_Stop = My.Resources.ICONSTOP
    '----------------------------------------
    InitTrayIcon() 'вывод иконки в трей с установкой версии 4
    '----------------------------------------
  End Sub

  Private Sub frm1_FormClosed(sender As Object, e As FormClosedEventArgs) Handles Me.FormClosed
    'this removes the icon from the system tray
    Dim nid As New NOTIFYICONDATA
    With nid
      .cbSize = IIf(sys_WinVista, Marshal.SizeOf(nid), Marshal.SizeOf(nid) - IntPtr.Size) 'NOTIFYICONDATA_V3_SIZE для XP
      .hWnd = Me.Handle
      .uID = vbNull
    End With
    Shell_NotifyIcon(NIM_DELETE, nid)

    IconTray_Start.Dispose()
    IconTray_Stop.Dispose()
  End Sub

  Private Sub ChangeTrayIcon(ByVal m_icon As Icon)
    'код вывода иконки в трей
    Dim nid As New NOTIFYICONDATA
    With nid
      .cbSize = IIf(sys_WinVista, Marshal.SizeOf(nid), Marshal.SizeOf(nid) - IntPtr.Size) 'NOTIFYICONDATA_V3_SIZE для XP
      .hWnd = Me.Handle
      .uID = vbNull
      .uFlags = NIF_ICON
      .hIcon = m_icon.Handle
      Debug.Print(m_icon.Handle)
    End With
    Shell_NotifyIcon(NIM_MODIFY, nid) 'меняем иконку в трее
  End Sub

'тест раз в секунду
  Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
    bTest = Not bTest
    If bTest Then
      ChangeTrayIcon(IconTray_Stop)
    Else
      ChangeTrayIcon(IconTray_Start)
    End If
  End Sub