ASP.NET

System.Text.StringBuilder sb = new System.Text.StringBuilder();
   sb.Append("<script language='javascript'>");
   sb.Append("window.open('javascript.htm', 'CustomPopUp',");
   sb.Append("'width=200, height=200, menubar=yes, resizable=no');<");
   sb.Append("/script>");

   //register with ClientScript
   //The RegisterStartupScript method is also slightly different
   //from ASP.NET 1.x
   Type t = this.GetType();
   if (!ClientScript.IsClientScriptBlockRegistered(t, "PopupScript"))
     ClientScript.RegisterClientScriptBlock(t,"PopupScript", sb.ToString());