Javascript counter reset

I have a script that counds back from X number of secounds, when the count reaches 0 a layer is displayed with a button. When the button in the layer is clicked the counter shoud startover and the layer should be hitten again, and only be displayed again when the counter again reaches 0. Can anybody help me, it should be easy, but everything i have tried haven't been working...

<input type="text" id="counter_advarsel">
<script>
function reset_counter() {
//here i need to reset the counter and hide the popup layer!!!???    
   
   
}
var countdownfrom=5;
var currentsecond=document.getElementById('counter_advarsel').value=countdownfrom+1;

function count_advarsel_function() {
if(currentsecond != 1) {
currentsecond -= 1;
document.getElementById('counter_advarsel').value=currentsecond;
} else {
document.getElementById('counter_div_advarsel').style.visibility = "visible";
document.getElementById('counter_div_advarsel').style.filter='progid:DXImageTransform.Microsoft.Shadow(color=#B5BDCB, direction=135, strength=6)';
document.getElementById('counter_div_advarsel').style.left = (winW/2)-(300/2);
document.getElementById('counter_div_advarsel').style.top = (winH/2)-(300/2);
}
setTimeout("count_advarsel_function()",1000);
}

document.onload = count_advarsel_function();
</script>
<?php
echo "<div style=\"z-index: 8000; position: absolute; visibility:hidden; top: -1000; left: -1000;\" id=\"counter_div_advarsel\">";
            echo "<table style=\"background-color: #ffffff; border: 1px solid #B5BDCB; width: 300px; height: 100px;\">";
            echo "<tr><td style=\"padding-left: 8px; padding-top: 4px; font-weight: bold; height: 24px;\" class=\"BorderBottom\">Automatisk log af advarsel</td></tr>";
            echo "<tr><td style=\"height: 60px; padding-left: 8px; padding-right: 8px; padding-bottom: 8px;\" class=\"BorderBottom\">";
            echo "<li>Du har nu været inaktiv i 18 minutter og vil, af sikkerhedsmæssige hensyn, defor automatisk blive logget af om 2 minutter.</li><br /><li>Gem alt igangværende arbejde eller genoptag brugen af systemet.</li>";
            echo "</td></tr>";
            echo "<tr><td style=\"text-align: center; height: 40px; vertical-align: middle;\">";
            echo "<input class=\"Knap\" type=\"button\" name=\"ok\" value=\"Ok\" style=\"width: 90px;\" onclick=\"javascript: reset_counter();\" ".$_SESSION["knap_opsaetning"]." />";                              
            echo "</td></tr>";
            echo "</table>";
echo "</div>";
?>