horizontal menu moves last link under when press ctrl + or -

Hi i have this horizontal menu, the problem with it is that if the user screen is to small or big for example if you minimize the page with (ctrl - )or ( ctrl +) you noticed that the last link on the menu goes under the bar is there a way that if the page is been viewed small shrink or enlarge the links as much as possible so no matter what the last links always fits on the bar? i thought about removing some links but i need all the links would this be the only solution??
this is my page
http://eurico.co.uk/1st4film/gallery.html

and this is my horizontal 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>

Code (markup):

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 7px;
	display: block;
	color: black;
	text-decoration: none;
}
ul#topnav li:hover {
	text-decoration:underline;
	border-bottom-color: red;

}
#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;}

Code (markup):