Problema con ie6 y css (probablemente con float)

body {
font-family: Verdana, Arial, Helvetica, Helv, sans-serif;
font-size: 12px;
}

a:active {
color: #ec8643;
}

a, a:visited {
color: red;
}

h1 {
font-size: 15px;
margin: 0;
}

h2 {
margin-top: 0;
color: #ec8643;
text-decoration: underline;
font-weight: bolder;
font-size: 18px;
text-align: left;
}

h3 {
font-size: 16px;
font-weight: 700;
}

h4 {
font-size: 12px;
font-weight: 400;
text-align: right;
}

em {
text-decoration: underline;
font-style: normal;
}

#contenedor {
margin: 0 auto;
width: 800px;
height: auto;
}

#barra_lateral {
width: 200px;
height: 600px;
float: left;
}

#barra_lateral p {
font-size: 18px;
margin-top: 30px;
}

#barra_lateral ul {
margin-top: 20px;
}

#barra_lateral li {
font-size: 15px;
list-style-type: square;
}

#barra_lateral a {
color: black;
line-height: 165%;
text-decoration: none;
}

#barra_lateral img {
z-index: -1;
position: absolute;
margin-top: 50px;
}

#barra_horizontal {
border-bottom-style: solid;
border-bottom-color: #ec8643;
background-color: #ec8643;
width: 800px;
text-align: right;
}

#barra_horizontal a {
color: #fff;
font-size: 17px;
font-weight: bolder;
margin-right: 12px;
text-decoration: none;
}

#barra_horizontal a:hover, #barra_lateral a:hover {
text-decoration: underline;
}

#barra_inferior {
border-bottom-style: solid;
border-bottom-color: #ec8643;
background-color: #ec8643;
width: 800px;
text-align: center;
margin-top: 630px;
border-top-style: solid;
border-top-color: #ec8643;
border-width: 6px;
color: #fff;
font-size: 12px;
}

#enlaces_inferiores {
width: 800px;
text-align: center;
margin-top: 6px;
margin-bottom: 10px;
}

#enlaces_inferiores a {
margin: 0 4px;
}

#columna_uno {
width: 290px;
margin: 0 0 0 210px;
height: 600px;
position: absolute;
}

#columna_uno img {
float: left;
margin: 0 10px 10px 0;
border: 3px solid #ec8643;
height: auto;
width: auto;
}

#columna_uno_sector2 {
padding-top: 10px;
border-top: 1px dashed #ec8643;
}

#columna_dos {
width: 280px;
margin: 0 0 0 20px;
float: right;
height: 583px;
}

#columna_dos img {
float: right;
margin-right: 10px;
border: 3px solid #ec8643;
height: auto;
width: auto;
}

#columna_dos_sector2 {
width: 280px;
padding-top: 10px;
position: absolute;
text-align: justify;
border-top: 1px dashed #ec8643;
}

#columna_dos_sector2 img {
float: left;
}

#imagen_sinborde img {
border: 0;
}

#columna_dos_sector3 {
margin-top: 210px;
width: 280px;
padding-top: 10px;
position: absolute;
border-top: 1px dashed #ec8643;
}