Declarar en un módulo:

Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName_
        As String, ByVal lpWindowName As String) As Long

Declare Function SetWindowPos Lib "user32" (ByVal hwnd As Long, ByVal hWndInsertAfter_
        As Long, ByVal x As Long, ByVal y As Long, ByVal cx As Long, ByVal cy As Long,_
        ByVal wFlags As Long) As Long

Global Ventana As Long
Global Const Mostrar = &H40
Global Const Ocultar = &H80

En un formulario :

'Oculta la barra de tareas
Ventana = FindWindow("Shell_traywnd", "")
Call SetWindowPos(Ventana, 0, 0, 0, 0, 0, Ocultar)

'Muestra la barra de tareas
Call SetWindowPos(Ventana, 0, 0, 0, 0, 0, Mostrar)