Вставлять в HTML-низ.

<script language="javascript">
function showldiv()
{
if(  document.getelementbyid("logindiv").style.display == 'none' )
     document.getelementbyid("logindiv").style.display = ''
else document.getelementbyid("logindiv").style.display = 'none'

return false;
}
function check_form()
{
if(document.forms["form_login"].req_username.value=='' || document.forms["form_login"].req_username.value=='') {
alert('оба поля обязательны для заполнения');
return false;
}
return true
}
url=document.url
if(url.indexof("login.php")==-1  && document.getelementbyid('navlogin')){
document.getelementbyid('navlogin').innerhtml  = "<a href=\"login.php\" onclick=\"return showldiv()\"><span>войти</span></a>"
login = "<center><div id=logindiv style=\"display: none; \">"

login += "<form id=form_login name=login method=post action=\"login.php?action=in\" onsubmit=\"return check_form()\">"
login += "<fieldset>"
login += "<legend>введите ваше имя и пароль ниже</legend>"
login += "<input type=hidden name=form_sent value=1 />"
login = login + "<input type=hidden name=redirect_url value="+document.url+" />"
login += "<br><table width=\"100%\" align=left>"
login += "<tr><td align=left><b>имя :</b></td><td align=left><input type=\"text\" id=\"fld1\" name=\"req_username\" size=\"30\" maxlength=\"25\" /></td></tr>"
login += "<tr><td align=left><b>пароль :</b></td><td align=left><input type=\"password\" id=\"fld2\" name=\"req_password\" size=\"30\" maxlength=\"16\" /></td></tr>"
login += "</table>"
login += "</fieldset>"
login += "<p class=formsubmit><input type=submit class=button name=login value=\"войти\" /><br /> <a href=\"register.php\">еще не зарегистрированы?</a><br /> <a href=\"login.php?action=forget\">забыли пароль?</a></p>"
login += "</form>"
login += "</div></center>"

document.getelementbyid('pun-navlinks').innerhtml += login
}
</script>

<style>
#logindiv{
    position:absolute;
    z-index: 2;
    background-color : gray;
    border : solid;
    padding-bottom : 5px;
    padding-left : 5px;
    padding-right : 5px;
    padding-top : 5px;
    width: 300px;
}
</style>