The quick and dirty way:
1) Drop the shiv file (html5shiv.js) in the wp-includes/js folder
2) Open wp-content/themes/fiver/header.php and add the following code somewhere before the CSS call (so maybe, right after the line with <HEAD>):
And check if that works.
Adding the HTML5 Shiv will not make IE magically show all kinds of CSS3 effects like shadows and rounded corners, though. It will only make IE “recognize” HTML5-based tags such as HEADER, FOOTER, SECTION, etc. instead of ignoring them or treat them as inline elements.
For the record, the subject of this thread is incorrect. It’s not IE messing with your layout, it’s you messing with IE. :)