#header #main_links {
background: #000;
height: 50px;
width: 780px;
}

#header #main_links li {
display: inline;
padding: 17px;
}

#header #main_links li a {
color: #fff;
}

#header #main_links li a:hover {
text-decoration: underline;
}

--

    <div id="header">
      <div class="logo">
        <a href="http://www.mystore.com"><img src="images/logo.gif" alt="MyStore"></a>
      </div>
      <ul id="main_links">
        <li><a href="<?php echo $prefix;?>index.php">Home</a></li>
        <li><a href="<?php echo $prefix;?>women.php">Women</a></li>
        <li><a href="<?php echo $prefix;?>men.php">Men</a></li>
        <li><a href="<?php echo $prefix;?>babies_and_kids.php">Babies & kids</a></li>
        <li><a href="<?php echo $prefix;?>health_and_beauty.php">Health & beauty</a></li>
        <li><a href="<?php echo $prefix;?>at_home_and_gifts.php">At Home & gifts</a></li>
        <li><a href="<?php echo $prefix;?>books_and_knowledge.php">Books & knowledge</a></li>
      </ul