Public Sub WorkbookActivate(ByVal FileName As String)
  On Error Resume Next
 
  Dim I As Long
  Dim sPath As String, sFile As String
 
  If FileName = "" Then
    MsgBox "Kein Dateiname übergeben!", vbExclamation
    Exit Sub
  End If
 
  sPath = ""
  sFile = ""
 
  For I = Len(FileName) To 1 Step -1
    If Mid(FileName, I, 1) = "\" Then
      sPath = Left(FileName, I)
      sFile = Mid(FileName, I + 1, Len(FileName))
      Exit For
    End If
  Next I
 
  If sPath = "" Then
    MsgBox "Sie haben keinen vollständigen Dateipfad übergeben!" & _
      vbNewLine & FileName, vbExclamation
    Exit Sub
  End If
 
  On Error Resume Next
 
  Workbooks(sFile).Activate
 
  If Err.Number <> 0 Then
    On Error Resume Next
    Workbooks.Open FileName:=FileName
 
    If Err.Number <> 0 Then
      MsgBox "Die Arbeitsmappe " & sFile & _
        " konnte nicht geöffnet werden!" & vbNewLine & FileName & _
        vbNewLine & vbNewLine & "Fehlernummer: " & Err.Number & _
        vbNewLine & "Fehlerbeschreibung: " & Err.Description, _
        vbExclamation
    End If
  End If
 
End Sub

WorkbookActivate "C:\Mappe1.xls"

Private Sub CommandButton1_Click()
  WorkbookActivate "C:\Mappe1.xls"
End Sub

Viel Spass,
R@lf