To add Javascript to Standard Pagelayout in Salesforce, add a HTML Home page Component with HTML checkbox checked.

Sample Javascript:

<style type="text/css">
#popup
{
display:none;
position:absolute;
height:100px;
width:100px;
background:#E6E6FA;     
border: 2px solid black;
overflow:auto;
}
#cls
{
color:red;
text-align:right;
}
</style>

<script type="text/javascript" src="/soap/ajax/22.0/connection.js"></script>

<script src="http://jqueryjs.googlecode.com/files/jquery-1.2.6.min.js" type="text/javascript"></script>

<script type="text/javascript">
    $(document).ready( function(){

        $(".dataCell").click(function(event){   
            $("#popup").css( {position: "absolute", top:event.pageY - 110, left: event.pageX});
            var temp = $(this).prev(".dataCell").find("a").attr("href");
            var depLink = temp.substring(1);           
            var qryString = "SELECT Description__c FROM Dependent__c where Id = '" + depLink + "'";
            sforce.connection.sessionId = getCookie('sid');
            var soql = qryString.toString();
           
            var result = sforce.connection.query(soql);   
           
            var records = result.getArray("records");
            for (var i = 0; i < records.length; i++)
            {
                var taskRecord = records[i];
                $("#cont").html(taskRecord.Description__c);
            }           
           
            showBox();
        });

        $("#cls").click(function(){
            hideBox();
        });

        function showBox()
        {
            $('#popup').fadeIn("slow");
        }
        function hideBox()
        {
            $('#popup').fadeOut("slow");
        }
    });
</script>
<div id="popup">
    <div id="cls">
    <a href="#" style="color:red;">X</a><br/>
    </div>
    <div id="cont"></div>
</div>

Output: