How to navigate with keyboard through <ul> <li> element

Possible Duplicate:
KeyBoard Navigation for menu using jquery

I created a menu using <ul> <li> tags and showing it to the user when he presses Enter key in the textbox. He can select items of the menu (navigate in menu) using mouse but I want also to allow him to select items from that menu using up/dow buttons of the keyboard for example.

Is it any way to do that using jQuery or CSS?

My menu has following structure:

<div class="services">
  <div class="items">
    <ul>                           
        <li class="mail-icon"><a href="#" id="mail"><?php echo $langmsg['mail']; ?></a></li>
        <li class="forum-icon"><a href="#" id="forum"><?php echo $langmsg['forum']; ?></a></li>
        <li class="chat-icon"><a href="#" id="chat"><?php echo $langmsg['chat']; ?></a></li>
    </ul>
  </div>
</div>

Note: <li> element has a background image also.