///我这里直接禁用了button
 <asp:ImageButton ID="imgbtnClear" runat="server" 
                     ImageUrl="~/image/shouye/qinkong.png" onclick="imgbtnClear_Click"  Enabled="false"/>

js:从网上下载的插件
$(function(){
jQuery.fn.countDown = function(settings) {
    
        var cd=new Date();
    
        var td=new Date(settings.startDate);    
        
        var displayText="";
        
        //开始构造显示用的字串
        
        var secondDiff=cd.dateDiff("s",td);
        
        var dayDiff=parseInt(secondDiff/(60*60*24));
        displayText+=(dayDiff>=0?dayDiff:"0")+"天 ";
        
        var hourDiff=parseInt(secondDiff/(60*60));
        displayText+=(hourDiff>=0?hourDiff%24:"0")+"小时 ";
        
        var minuterDiff=parseInt(secondDiff/(60));
        displayText+=(minuterDiff>=0?minuterDiff%60:"0")+"分 ";
        
        
        displayText+=(secondDiff>=0?secondDiff%60:"0")+"秒";
    
    $(this).text(displayText).animate({"none":"none"},settings.duration,'',function() {
        if(secondDiff>=0) {
//在这里，如果时间大于0，则启用按钮（难道我这个移除禁用按钮的代码写错了？？？求各位大神赐教）
        jQuery("#btnTouzhu").removeAttr("disabled");
            $(this).countDown(settings);
            
        }else{
//如果时间小于等于0，按钮失效，这里可以
            $("#imgTouzhu").attr({src:"../../image/shouye/ddkj.gif"});
            $("#imgTouzhu").attr({"disabled":"disabled"});

            settings.callBack(this);
        }
        
    });        
    return this;
};