#menu {
width: 12em; /* set width of menu */
background: #eee;
} 

#menu ul { /* remove bullets and list indents */
list-style: none;
margin: 0;
padding: 0;
}

/* style, color and size links and headings to suit */
#menu a, #menu h2 {
font: bold 11px/16px arial, helvetica, sans-serif;
display: block;
border-width: 1px;
border-style: solid;
border-color: #ccc #888 #555 #bbb;
margin: 0;
padding: 2px 3px;
}

#menu h2 {
color: #fff;
background: #0000A0;
text-transform: uppercase;
}

#menu a {
color: #000;
background: #efefef;
text-decoration: none;
}

#menu a:hover {
color: #a00;
background: #fff;
}

#menu li {
/* make the list elements a containing block for the nested lists */
position: relative;
} 

#menu ul ul ul {
position: absolute;
top: 0;
left: 100%; /* to position them to the right of their containing block */
width: 100%; /* width is based on the containing block */
}

div#menu ul ul ul,
div#menu ul ul li:hover ul ul
{display: none;}

div#menu ul ul li:hover ul,
div#menu ul ul ul li:hover ul
{display: block;}

<!--[if IE]>
<style type="text/css" media="screen">
body {
behavior: url(csshover.htc); /* call hover behaviour file */
font-size: 100%; /* enable IE to resize em fonts */
} 
#menu ul li {
float: left; /* cure IE5.x "whitespace in lists" problem */
width: 100%;
}
#menu ul li a {
height: 1%; /* make links honour display: block; properly */
} 

#menu a, #menu h2 {
font: bold 0.7em/1.4em arial, helvetica, sans-serif; 
/* if required use em's for IE as it won't resize pixels */
} 
<![endif]-->