if($.browser.msie && $.browser.version==7){
//ie7下执行该区域代码
}

<script>
var browser = (function(){
var isIE6 = /msie 6/i.test(navigator.userAgent);
var isIE7 = /msie 7/i.test(navigator.userAgent);
var isIE8 = /msie 8/i.test(navigator.userAgent);
var isIE = /msie/i.test(navigator.userAgent);
return {
msie:isIE,
version:function(){
switch(true){
case isIE6:return 6;
case isIE7:return 7;
case isIE8:return 8;
}
}()
};
})();
alert(browser.msie);
alert(browser.version);
</script>