Unable to Close Popup window using ASP.Net

I am using Asp.net web application.....

I am having a problem closing a popup window opened modally. When I
try to close the window (when the user hits Search button and the data
has been processed), the Popup window opens as a full screen as a new
window. The original window plus the Modally opened Pop remain in a
separate window.

What I want to do is close the popup and return to the original window
with its view state maintained.

The control use to fire the popup window and the one to save the
changes need to be a web server control.
The popup window needs to be modal.

here i am using show modal dialog......
************************************************** *************************
The code I am using open the window with this control is:

Main.aspx.cs
=========

protected void Button1_Click(object sender, EventArgs e)
        {
         
            string url = "window.showModalDialog('WebForm2.aspx',null,'font-size:10px;dialogWidth:43em;dialogHeight:40em,toolbar=0,location=0,directories=0,status=no,menuB ar=1,scrollBars=no,resizable=no')";
           
            if (!ClientScript.IsClientScriptBlockRegistered("OpenPopup"))
                ClientScript.RegisterClientScriptBlock(typeof(string), "OpenPopup", url, true);
        }  


************************************************** *************************


Sub.aspx
=======

 protected void Button4_Click(object sender, EventArgs e)
        {

            Response.Write("");
            Response.End();
         
  //Response.Write("");    [this code one of option ]

            //string script = "this.window.opener.location=this.window.opener.location;this.window.close();"; [this code one of option ]
            //if (!ClientScript.IsClientScriptBlockRegistered("REFRESH_PARENT"))
            //    ClientScript.RegisterClientScriptBlock(typeof(string), "REFRESH_PARENT", script, true);
        }


What ever action i did in the sub.aspx[modal popup] will open new full screen




Hopefully someone has a solution in this regard.

Cheers.

Kumar