Наследование: System.Windows.Forms.Form
 private void accountManagerToolStripMenuItem_Click(object sender, EventArgs e)
 {
     Cursor = Cursors.WaitCursor;
     frmAccounts fAccounts = new frmAccounts();
     fAccounts.MdiParent = this;
     fAccounts.Show();
     Cursor = Cursors.Default;
 }