menu position wrong in IE; good in FF

If you view https://www.acctv.org with IE, the top nav menu displays as a vertical list on the right side and the posts and two sidebars are pushed down. If you select one of the ‘page’ links the content and sidebars are in the correct position, but the top nav menu is now partially covering the far right sidebar.

If you view the site home page with FireFox, all is well.

Seems like an index.php and CSS issue?

Can anyone give me some tips on how to resolve this? (Keep in mind I am not a developer; more user with experience setting up WP sites).

Thanks
Gary