The error you mention occurs in IE6 but not in Firefox; however, there are header element spacing/overlap errors in Firefox that do not occur in IE.
Your page HTML does not validate (W3C Markup Validation Service ).
Part of the problem is that you have the leaderboard code before the doctype comment and before the opening <html> tag. This is causing many many other errors.
HTML Code:
<center><!-- Start Leaderboard - 728x90 Code -->
<script src="http://cds.adecn.com/add/script.js?v=2.2;siteId=66343;spotId=11783;width=728;height=90">
</script>
<!-- End Leaderboard - 728x90 Code --></center>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" dir="ltr" lang="nl">
<head>
<title>Noveenkaarsen : Zen Cart!, The Art of E-commerce</title>
You need to relocate that code first, and then see what other errors there may be.