how can i stop last link on horizontal menu from float out

[Lisa23]

Hi i have the thsi menu whihc can be seen on the link above my problems is if you minimize the page with (ctrl -) or mximize (ctrl +) the last link text (contact us) moves out the menu bar is there a way that i can prevent that from happening??

 

http://eurico.co.uk/1st4film/gallery.html

 

code HTML

<ul id="topnav">
        <li><a href="index.php"><b>HOME</b></a></li>
        <li><a href="home2.html"> <b>ABOUT US</b></a></li>
        <li><a href="gallery.html"><b>GALLERY</b></a></li>
        <li><a href="market.php"><b>VIDEO MARKETING</b></a></li>
        <li><a href="#"><b>WEB <span style="color: #0F0; font-weight:900px;">TV</span> PRESENTERS.co.uk</b></a></li>
        <li><a href="#"><b>AWARDS</b></a></li>
        <li><a href="#"><b>INSTANT QUOTE</b></a></li>
        <li><a href="#"><b>EQUIPMENT</b></a></li>
        <li><a href="#"><b>STAFF</b></a></li>
        <li><a href="#"><b>CONTACT US</b></a></li>      
               
    </ul>

 

 

CSS

ul#topnav {
margin: 0; padding: 0;
float: left;
width: 800px;
height:25px;
list-style: none;
position: absolute;
font-size: 1.2em;
background-image:url(images_menu_top/middle.gif);


}
ul#topnav li {
float: left;
margin: 0; padding: 0;
border-left: 1px solid #555;
height:25px;
}
ul#topnav li a {
padding: 5px 5px;
display: block;
color: black;
text-decoration: none;
}
ul#topnav li:hover {
text-decoration:underline;

}
#topnav li ul {
float: left;
padding: 10px 0;
position: absolute;
left: 0; top:25px;
display: none;
width: 1000px;
color: orange;
-moz-border-radius-bottomright: 5px;
-khtml-border-radius-bottomright: 5px;
-webkit-border-bottom-right-radius: 5px;
-moz-border-radius-bottomleft: 5px;
-khtml-border-radius-bottomleft: 5px;
-webkit-border-bottom-left-radius: 5px;
}
#topnav li:hover ul { display: block; }
#topnav li ul a { display: inline; color:orange; }
#topnav li ul a:hover {text-decoration: underline;}