滚屏弹出固定

//ie6 position fixed bug fix

                  var vs=window.navigator.appVersion;

                  if (vs.indexOf( 'MSIE 6' )!=-1) {     

                      document.getElementById( 'pop' ).style.top=(document.documentElement.scrollTop+(window.screen.availHeight)/2)+ 'px' ;

                     window.onscroll= function () {

                         document.getElementById( 'pop' ).style.top=(document.documentElement.scrollTop+(window.screen.availHeight)/2)+ 'px' ;