Do you know why it would look fine on mine but different on someones elses? Even though the browser is the same version?
Hm, it could be some setting they have ... but I tested in two versions of IE7 and it was fine. Mind you, I've totally abandoned that out of date browser, and don't consider it worth designing for any more.
IE7 is an old browser eat up with Microsoft-peculiar behavior. I'd like to recommend that you validate the html and make it shine, THEN we can look at the css if a problem still exists. For example, <nav> is not supported in IE8 or earlier, so you should use a <div> instead. Something this simple could be the culprit. In the footer, there is an unmatched (stray) </ul> tag. That, too, might be the bad actor. Gotta start somewhere, and clean HTML is a good foundation.
Another thing, too. IE7 does not support an XHTML strict doctype, so you would be much better off using an HTML401 doctype. That's a biggie.