To reproduce: disable Aero on a Windows7 machine and use the following code:

public Form1()
{
    InitializeComponent();

    this.IsMdiContainer = true;

    Form MSform = new Form();
    MSform.Text = "Form MDI Child 1";
    MSform.MdiParent = this;
    MSform.Show();

    MSform = new Form();
    MSform.Text = "Form MDI Child 2";
    MSform.MdiParent = this;
    MSform.Show();
    
    MSform = new Form();
    MSform.Text = "Form MDI Child 3";
    MSform.MdiParent = this;
    MSform.Show();

    RadForm form = new RadForm();
    form.Text = "RadForm MDI Child 1";
    form.MdiParent = this;
    form.Show();

    form = new RadForm();
    form.Text = " RadFormMDI Child 2";
    form.MdiParent = this;
    form.Show();
    
    form = new RadForm();
    form.Text = " RadFormMDI Child 3";
    form.MdiParent = this;
    form.Show();
}

private void Form1_Shown(object sender, EventArgs e)
{
    this.LayoutMdi(MdiLayout.TileHorizontal);
}

private void radButton1_Click(object sender, EventArgs e)
{
    this.LayoutMdi(MdiLayout.TileVertical);
}

private void radButton2_Click(object sender, EventArgs e)
{
    this.LayoutMdi(MdiLayout.Cascade);
}

private void radButton3_Click(object sender, EventArgs e)
{
    this.LayoutMdi(MdiLayout.TileHorizontal);
}

Workaround: set the RadForm.AllowTheming property to false.

Workaround 2:  public class MyForm : RadForm
 {
     protected override CreateParams CreateParams
     {
         get
         {
             CreateParams par = base.CreateParams;
             par.Style |= NativeMethods.WS_CAPTION;
             return par;
         }
     }
 }