Hi

on the site http://radio.rockt.es the menu bar look great in Firefox, but in IE6 it's not so good.

Here is the css code for the nav bar:

.nav {
	background:url(../img/menu-bar.png) no-repeat;
	margin-top: 25px;
	height:538px;
}
.nav ul {
	margin:0px 0 0 -25px;
}
.nav li {
	display:block;
	margin:-15px 0;
	height:49px;
	width:130px;
	background:url(../img/button-2.png) no-repeat;
}
.nav li:hover {
	background:url(../img/button-1.png) no-repeat;
	height:49px;
	width:130px;
}
.nav p {
	margin:0px;
	padding:18px 0 0 0px;
	text-decoration:none;
	color:#EAF83A;
	font-size:11px;
	font-weight:bold;
	text-align:center;
}

.nav p:hover {
	color:#111;
}

First thing to do is VALIDATE your code.
Second thing is to get rid of the tables for layout.
Third, you can read all about IE's numerous bugs here:
http://www.positioniseverything.net