I assume you have disabled all the addons. If that is the case, then one of the addons probably deals with the missing part of your window.
What I would do is (assuming you have XP):
1. Save the Favorites by exporting them to the Desktop.
2. Reinstall IE from
Download details: Windows Internet Explorer 7 for Windows XP SP2
3. Import the Favorites.
This is the easiest way to restore your IE.