'Esta API copiala en un modulo:

Declare Function ShellExecute Lib "shell32.dll" Alias "ShellExecuteA" (ByVal hwnd As Long, ByVal lpOperation As String, ByVal lpFile As String, ByVal lpParameters As String, ByVal lpDirectory As String, ByVal nShowCmd As Long) As Long
Public Const SW_SHOW = 1


'Esto en el botón de comando del Formulario

Private Sub CmdAbreFichero_Click()
'Esto en el formulario
If IsNull(Me.TxtCampo) = True Then
 MsgBox "Por favor introduzca ruta completa del fichero a cagar", vbInformation + vbOKOnly, "AVISO"
 Exit Sub
End If
ShellExecute Me.hwnd, "open", Me.TxtCampo, "", "", SW_SHOW
End Sub