IE 6: Revealing All Onload

I'm new to DOM and javascripting and have constructed a hide/show menu. I can't find a way to keep IE6 from revealing all the sub-lists onload. The code (inspired by PPK and Jeremy Keith's recent books) follows:

window.onload = function () {
var topnav = document.getElementById ("navmain");
var parent = document.getElementsByTagName("ul");
for (var i=0;i<parent.length;i++) {
if (parent[i].className != "section") continue;
parent[i].style.display = "none";
var header = parent[i].previousSibling;
if (header.nodeType != 1)
header = header.previousSibling;
header.relatedTag = parent[i];
header.onclick = openClose;
}
}
function openClose () {
var currentValue = this.relatedTag.style.display;
var newValue = (currentValue == "none") ? "block" : "none";
this.relatedTag.style.display = newValue;
}
Markup:
<div id="navmain" class="clearfix">
<ul><li><h4>Markets</h4>
<ul class="section">
<li><a href="/corporate/">Corporate</a></li>
<li><a href="/govt/">Govt/Agency</a></li>
<li><a href="/municipal/">Municipal</a></li>
<li><a href="/mbsabs/">MBS/ABS/CDO</a></li>
<li><a href="/funding/">Funding</a></li>
</ul>
</li></ul>
</div>

Hoping for some simple solution, but all advice is welcome.