$(document).ready(function() {

$("ul#topnav li").hover(function() { //Hover over event on list item
    $(this).css({ 'background' : '#1376c9 url(/images/topnav_active.gif) repeat-x'}); //Add background color + image on hovered list item
    $(this).find("span.menu_links").show(); //Show the subnav
} , function() { //on hover out...
    $(this).css({ 'background' : 'none'}); //Ditch the background
    $(this).find("span.menu_links").hide(); //Hide the subnav
});

});