把父窗体的IsMdiContainer属性改为true;

你看看这个程序能不能给你点启发.

完了,上传失败,我把代码贴上吧,Form2是一个原始窗体,什么都没改过的
using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;

namespace WindowsApplication6
{
/// <summary>
/// Form1 的摘要说明。
/// </summary>
public class Form1 : System.Windows.Forms.Form
{
private System.Windows.Forms.MainMenu mainMenu1;
private System.Windows.Forms.MenuItem menuItem1;
private System.Windows.Forms.MenuItem menuItem2;
private System.Windows.Forms.MenuItem menuItem3;
private System.Windows.Forms.MenuItem menuItem4;
private System.Windows.Forms.MenuItem menuItem_open;
private int num=1;
/// <summary>
/// 必需的设计器变量。
/// </summary>
private System.ComponentModel.Container components = null;

public Form1()
{
//
// Windows 窗体设计器支持所必需的
//
InitializeComponent();

//
// TODO: 在 InitializeComponent 调用后添加任何构造函数代码
//
}

/// <summary>
/// 清理所有正在使用的资源。
/// </summary>
protected override void Dispose( bool disposing )
{
if( disposing )
{
if (components != null)
{
components.Dispose();
}
}
base.Dispose( disposing );
}

#region Windows 窗体设计器生成的代码
/// <summary>
/// 设计器支持所需的方法 - 不要使用代码编辑器修改
/// 此方法的内容。
/// </summary>
private void InitializeComponent()
{
this.mainMenu1 = new System.Windows.Forms.MainMenu();
this.menuItem1 = new System.Windows.Forms.MenuItem();
this.menuItem2 = new System.Windows.Forms.MenuItem();
this.menuItem3 = new System.Windows.Forms.MenuItem();
this.menuItem4 = new System.Windows.Forms.MenuItem();
this.menuItem_open = new System.Windows.Forms.MenuItem();
//
// mainMenu1
//
this.mainMenu1.MenuItems.AddRange(new System.Windows.Forms.MenuItem[] {
this.menuItem1,
this.menuItem2,
this.menuItem3});
//
// menuItem1
//
this.menuItem1.Index = 0;
this.menuItem1.Text = "file";
//
// menuItem2
//
this.menuItem2.Index = 1;
this.menuItem2.MenuItems.AddRange(new System.Windows.Forms.MenuItem[] {
this.menuItem4});
this.menuItem2.Text = "edit";
//
// menuItem3
//
this.menuItem3.Index = 2;
this.menuItem3.MenuItems.AddRange(new System.Windows.Forms.MenuItem[] {
this.menuItem_open});
this.menuItem3.Text = "Window";
//
// menuItem4
//
this.menuItem4.Index = 0;
this.menuItem4.Text = "AddWindow";
this.menuItem4.Click += new System.EventHandler(this.menuItem4_Click);
//
// menuItem_open
//
this.menuItem_open.Index = 0;
this.menuItem_open.Text = "OpenWindow";
//
// Form1
//
this.AutoScaleBaseSize = new System.Drawing.Size(6, 14);
this.ClientSize = new System.Drawing.Size(576, 477);
this.IsMdiContainer = true;
this.Menu = this.mainMenu1;
this.Name = "Form1";
this.Text = "Form1";

}
#endregion

/// <summary>
/// 应用程序的主入口点。
/// </summary>
[STAThread]
static void Main()
{
Application.Run(new Form1());
}

private void menuItem4_Click(object sender, System.EventArgs e)
{
Form2 frm=new Form2();
frm.Text="win"+num.ToString();
num++;
frm.MdiParent=this;
frm.Show();
MenuItem[] mis=new MenuItem[this.menuItem_open.MenuItems.Count+1];
for(int i=0;i<mis.Length-1;i++)
{
mis[i]=this.menuItem_open.MenuItems[i];
}
MenuItem mi=new MenuItem();
mi.Text=frm.Text;
mis[mis.Length-1]=mi;
this.menuItem_open.MenuItems.Clear();
this.menuItem_open.MenuItems.AddRange(mis);
}
}
}