Positioning Weirdness in IE6 and IE7

I've got some positioning problems that I can't figure out. Can anyone help?

My website was working fine in IE7 and the current releases of Firefox and
Opera so I had a look at it in IE6 and found that it was messed up. The
major problem is that in IE6 the index, which was supposed to be on the
left, was on the right, OVERLAYING the content on its right side.

I finally resolved that problem in a way that allows both versions of IE6
and IE7 to work without breaking Opera and Firefox. Unfortunately, now
something else happens that I'm finding perplexing: as I adjust the screen
size in IE6 _and_ IE7, sometimes all of the navigation index _and_ the
content disappear entirely! And it's not just when the screen is very small
either. For instance, if you look at this page -
http://sfl.london.on.ca/index.shtml - with the browser maximized, it is
fine. But, if you narrow it to about 80% of the the full width, the page
goes blank and stays blank until the width is about 60% of the full width.
This weirdness doesn't happen with Firefox or Opera though.

Any idea what I'm doing wrong?

I'd be especially interested in learning how to diagnose this problem myself
so if you can tell me how you figured out what was wrong, I'd really
appreciate it. Don't feel you need to write a long essay though; just a
pointer to an article that describes the technique would be fine if you know
of such an article.

--

Rhino