<%@ Register TagPrefix="telerik" Namespace="Telerik.Web.UI" Assembly="Telerik.Web.UI" %>
<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
<telerik:RadWindow runat="server" ID="Window1" VisibleOnPageLoad="true">    
        <ContentTemplate>    
            <uc1:WebUserControl ID="WebUserControl1" runat="server" />    
        </ContentTemplate>    
    </telerik:RadWindow>   
 UserControl uc2 = (UserControl) LoadControl("~/Window/Content/WebUserControl2.ascx"); 
    Window1.ContentContainer.Controls.Clear(); 
    Window1.ContentContainer.Controls.Add(uc2);

--- WIndows with Close Icon Only
<telerik:RadWindow runat="server" ID="Window1" VisibleOnPageLoad="true" Modal="true" Behaviors="Close" >    
        <ContentTemplate>                
            <p>Hello World</p>
        </ContentTemplate>    
    </telerik:RadWindow>   

protected void ShowModalPopup_Load(object sender, EventArgs e)
        {
            this.Window1.VisibleOnPageLoad = true;
            //-VisibleOnPageLoad="true"
        }