IE showing drop-down menu as standard menu items

In FireFox this works fine, but testing in IE7 has shown that whatever changes I've made are causing problems. The only changes that I've made have been to the main CSS file of my template, and these are:

ul.nice-menu {
	padding: 0px;
	margin: 0px;
	width: 100%;
	background: url(submenu_main_back.gif) bottom left repeat-x;
	text-align: left;
	height: 28px;
	padding: 5px 0px 0px 0px;
	font-size: 12px;
}
ul.nice-menu ul a, ul.nice-menu li a {
	background: url(submenu_back.gif);
	background-position: top right;
	background-repeat: no-repeat;
	padding: 5px 25px 5px 10px;
	color: #FFFFFF;
	text-decoration: none;
	border: none;
}
ul.nice-menu ul a:hover, ul.nice-menu li a:hover {
	text-decoration: underline;
}
ul.nice-menu ul ul a, ul.nice-menu li li a {
	background:  #000000;
	padding: 2px;
	margin: 0px;
	border: none;
	height: 16px;
}
#block-nice_menus-1 div.title {
	display: none;
	visibility: hidden;
}
#menu-63, #menu-64, #menu-65, #menu-66, #menu-67 {
	border: none;
}
div#block-nice_menus-1 div.content {
	margin: 0px;
	padding: 0px;
}

As you can see by the image, Firefox is fine, but IE isn't. Is there a problem with the CSS that I've done that is causing this?