MSN Explorer is the equivalent of the AOL software you get when you sign up with them but with MSN. Both are, I believe, built around IE but with added in features. You can find an older copy here just for reference.
Edit: Here's the most recent version.
Actually when I pull up your blog in IE 6.0.2900.2180, I get the larger text and all that although the sidebar looks fine. The sidebar dropping is usually caused by something within one of your posts being too wide for the post area, like a picture or something. Nothing appears to be to wide though currently.
The issue is, and you'll find this with many sites, is that Microsoft does it's own interpretation of CSS. Some theme designers create a seperate CSS file just for IE because of this. You can see from your page source, this theme just has one CSS file called so, in this case, the designer didn't.
Heck, when I bring up the CSS file itself, Netscape displays it while IE tries to load Front Page.
When you bring up the CSS file, you'll note the font sizes are in either percentages or in 'em'. And that's where you run into your problem.
I've been sitting here trying to find a chart to show/ prove this but I can't.
The text size is determined by em settings in the CSS file. Unfortunely, the basis that the browsers start off with on em is larger in IE than it is in Netscape and the Greko based browsers.
I still can't find the chart that I saw a few days ago to prove this. Sorry about that. I did find a forum thread about it but it's dated in 2001. Says basically the same thing but the reasoning may be different now.
Best bet would be to suggest to the visitor that they adjust the text setting in their browser, View -> Text Size, and make adjustments there.
Until Microsoft fixes IE to be like the other browsers, there's not a whole lot that can be done unless we can get the theme redesigned.
Heck, try pulling up Google's home page in the two browsers. Big difference in those.
Hope this sort of helps,
reedit: Actually, the sidebar does drop when you go up and down in font size in IE. At Medium for me, it's fine. At large, it drops.