<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>

// JavaScript Document
var 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
    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();
?>

document.getElementById("button").disabled    = 'true';//禁用
document.getElementById("button").removeAttribute("disabled");//恢复

------解决方案--------------------

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
            }
        }
    }
}