I have this website, http://www.escuelita.info. Now I tried many times to make it work on IE 7 (possibly IE6 too didn't check) but for some reason the middle frame on the right doesn't show content.
If you compare it to Firefox or Chrome (or even IE8) it works ok. I need to fix this but I'm not sure what's causing the problem..
Note
I checked with IE7 and the pages don't show content
(the main content DIV content_outer)..
Your website is not valid XHTML. Use http://validator.w3.org/ to weed out errors before going any further. Errors affect rendering.
If the problem persists then one way to fix such issues is to "upgrade" IE with ie7-js. It's a JavaScript library that makes IE behave like a standards-compliant browser.
Using a CSS framework might help also, for example Tripoli; it gives you a consistent look over all major browsers.
The content shows below the sidebar. Either IE7 enters quirks mode or you trigger some sort of bug that causes IE7 to misbehave.
Double check your doctype, tags and most importantly, get rid of those nested tables. Nested tables are EVIL and I'd bet they're your culprit. Use CSS grids (here's one that's really good: http://www.1kbgrid.com/) instead.
Dunno if this solves your problem, hopefully so.