You will have to wrap both your banner and the navigation in a fixed position element
HTML(header only)
<header>
<span class="banner_h">
<img src="Images\Top_Banner_4.png" alt="Banner" height="150" width ="1240" />
</span>
<nav>
<ul class="nav">
<li><a href="index.html">Home</a></li>
<li><a href="about.html">About Us</a></li>
<li><a href="contact.html">Contact Us</a></li>
<li><a href="membership.html">Become a Member</a></li>
<li><a href="borrow.html">Borrow Toys</a></li>
<li><a href="policies.html">Our Policies</a></li>
<li><a href="sitemap.html">Site Map</a></li>
</ul>
</nav>
</header>
CSS:
header {
position:fixed;
top: 0;
left: 0;
width: 100%;
}