请问大师们,查询注册用户名已存在时按钮不提交，不跳转，如何实现

请教大师们,查询注册用户名已存在时按钮不提交，不跳转，怎么实现？

PHP code

  <form id="form1" onsubmit="return hostvalid();" method="post" name="form1" action="index.php?reg">          <div class="home_right01">            <div class="home_right01a"><input id="host" class="input01" name="host" onblur="checkname();" /></div>            <div id="msg" class="home_right01b">请输入名称</div>            <div class="home_right01c">              <div class="applyfor_btbg">                <input id="button" class="applyfor_bg" title="注册" value="" type="submit" name="button" />              </div>            </div>          </div>        </form>

JScript code

// JavaScript Documentvar XHR;function createXHR(){    if(window.ActiveXObject){        XHR=new ActiveXObject('Microsoft.XMLHTTP');    }else if(window.XMLHttpRequest){        XHR=new XMLHttpRequest();    }}function checkname(){    var username=document.form1.host.value;    createXHR();        XHR.open("GET","./checkname.php?id="+username,true);    XHR.onreadystatechange=byhongfei;    XHR.send(null);}function byhongfei(){    if(XHR.readyState == 4){        if(XHR.status == 200){                var textHTML=XHR.responseText;                        document.getElementById('msg').innerHTML=textHTML;        }    }}

PHP code

<?php    mysql_connect("localhost",'root','123456');    mysql_select_db('dodiscuz_freebb');    $sql="select * from member where username='$_GET[id]'";    $query=mysql_query($sql);    if(is_array(mysql_fetch_array($query))){        echo "用户名已存在";    }else{        echo "<font color=green>用户名可以使用</font>";    }mysql_close();?>

JScript code

document.getElementById("button").disabled    = 'true';//禁用document.getElementById("button").removeAttribute("disabled");//恢复
[解决办法]
JScript code
function byhongfei(){    if(XHR.readyState == 4){        if(XHR.status == 200){                var textHTML=XHR.responseText;                        document.getElementById('msg').innerHTML=textHTML;            if(textHTML == "用户名已存在"){              document.getElementById("button").disabled    = 'true';//禁用            }        }    }}