Metti in un modulo questo:
codice:
Option Explicit
Private 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
Private Const SW_NORMAL = 1
Public Sub ExecFile(ByVal fileName As String)
    Dim ret As Long
    ret = ShellExecute(0, "open", fileName, vbNullString, vbNullString, SW_NORMAL)
    If ret<=32 Then
        Err.Raise 51, "ExecFile", "Impossibile eseguire il file """ & fileName & """." & vbCrLf & "Codice di errore di ShellExecute: " & LTrim(CStr(ret))+"."
    End If
End Sub
, quindi per aprire un file usa ExecFile.