Private Sub ToolBar1_ButtonClick(ByVal sender As System.Object, ByVal e As _
  System.Windows.Forms.ToolBarButtonClickEventArgs) Handles ToolBar1.ButtonClick
        Select Case e.Button.ToolTipText
 
            Case "Anmelden"
                Dim frm_Log As New frmLogon()
                frm_Log.MdiParent = Me
                Me.ToolBar1.Buttons(0).Enabled = False
                frm_Log.Show()
 
 
        End Select
    End Sub

Private Sub cmd_OK_Click(ByVal sender As System.Object, ByVal e As _
  System.EventArgs) Handles cmd_OK.Click
        fMain = Me.ParentForm
        fMain.ToolBar1.Buttons(0).Enabled = True
        Me.Close()
    End Sub

VB RULEZ !