Html:

<nav class="navbar navbar-defalt" data-offset-top="120" data-spy="affix" >
    ...
</nav>

Css:

<style>
.navbar {
    background-color: red;
}
.navbar.affix {
    background-color: green;
}

</style>

<div class="container" id="con">
    <div class="row">
        <div class="span12">
            <div class="well">
                <h1> Header </h1>               
            </div>
        </div>
    </div>
</div>
<div class="container" data-spy="affix" data-offset-top="400" id="nav">
    <div class="navbar">
        <div class="navbar-inner">
            <div class="container">
                <div class="span12">
                    <a class="brand" href="#">Home</a>
                     <ul class="nav">
                        <li class="active"><a href="#">Home</a></li>
                        <li><a href="#">Link</a></li>
                        <li><a href="#">Link</a></li>
                     </ul>
                </div>
            </div> 
        </div> 
    </div> 
</div>
<div class="container">
    <div class="span3">
        <p style="height:1000px;padding:10px;">
          
            Long scrolling text here... Ovi lispmd idr. Blah goo bar foor foo.  Ovi lispmd idr. Blah goo bar foor foo. Ovi lispmd idr. Blah goo bar foor foo. Ovi lispmd idr. Blah goo bar foor foo. Ovi lispmd idr. Blah goo bar foor foo. Ovi lispmd idr. Blah goo bar foor foo. Ovi lispmd idr. Blah goo bar foor foo. Ovi lispmd idr. Blah goo bar foor foo. Ovi lispmd idr. Blah goo bar foor foo. Ovi lispmd idr. Blah goo bar foor foo.
            
        </p>
    </div>
</div>

<style>

#con .well {
    height:400px;
}

#nav.affix {
    position: fixed;
    top: 0;
    width: 100%
}
</style>

<script>
    $('#nav').affix();
</script