验证IE6.0和IE7.0

    function btn_selectuser_onclick()
    {
        var ua = navigator.userAgent;
  if(ua.lastIndexOf("MSIE 7.0")!=-1)
  {
     var myreturn = window.showModalDialog("../PublicPage/BuyUserChoose.aspx",'window',"dialogWidth=250px;dialogHeight=165px;center=yes;scroll=auto;status=no;");
           if(myreturn)
           {  
              var company = document.getElementById("txt_Username"); 
              company.value = myreturn;
           }
  }
  else if(ua.lastIndexOf("MSIE 6.0")!=-1)
  {
      var myreturn = window.showModalDialog("../PublicPage/BuyUserChoose.aspx",'window',"dialogWidth=240px;dialogHeight=200px;center=yes;scroll=auto;status=no;");
            if(myreturn)
            {  
              var company = document.getElementById("txt_Username"); 
                company.value = myreturn;
            }
  }    
    }