.mydiv {
background-color: #FFFFFF;
line-height: 40px;
font-size: 12px;
font-weight: bold;
z-index:999;
width: 503px;
left:50%;/*FF IE7*/
top:50%;/*FF IE7*/

margin-left:-200px!important;/*FF IE7 该值为本身宽的一半 */
margin-top:-230px!important;/*FF IE7 该值为本身高的一半*/
margin-top:0;
position:fixed!important;/*FF IE7*/
position:absolute;/*IE6*/

_top:       expression(eval(document.compatMode &&
            document.compatMode=='CSS1Compat') ?
            documentElement.scrollTop + (document.documentElement.clientHeight-this.offsetHeight)/2:/*IE6*/
            document.body.scrollTop + (document.body.clientHeight - this.clientHeight)/2);/*IE5 IE5.5*/

}


.bg {
z-index:998;
background-color: #666; 
display:none;
width:1280px;
height:1340px!important;/*FF IE7*/
height:1600px;/*IE6*/
left:0;
top:0;/*FF IE7*/
filter:alpha(opacity=50);/*IE*/
opacity:0.5;/*FF*/
position:fixed!important;/*FF IE7*/
position:absolute;/*IE6*/

_top:       expression(eval(document.compatMode &&
            document.compatMode=='CSS1Compat') ?
            documentElement.scrollTop + (document.documentElement.clientHeight-this.offsetHeight)/2;/*IE6*/
            /*document.body.scrollTop + (document.body.clientHeight - this.clientHeight)/2);IE5 IE5.5*/

}