You didn't say what version of Windows you're using, so I've tried to include the steps for both Windows 98 and Windows XP. However, you may see some steps that aren't quite right because of the many variables you can choose. It is SO much easier to answer these questions if you include the basic information about your system.
I think you can just uninstall, then reinstall Microsoft Outlook Express which should repair it.
Click Start/Settings/Control Panel (or just Start/Control Panel) and then open the Add-Remove Programs utility.
Scroll to the Microsoft Outlook Express entry and select it. Click the Add-Remove button (or the Change/Remove button) and uninstall Outlook Express.
You should be prompted to restart the computer, then when it restarts, go back to the Add-Remove Programs utility.
Scroll to the Microsoft Internet Explorer entry and select it, then click the Add-Remove button (or the Change/Remove button).
You should see a Window similar to this:

Yours should not appear "grayed out" as it does in the picture.
Anyway, click the "Add a component" option. Check Outlook Express to reinstall it.
If you see a dialog box asking for the installation CD, try directing it to the C:\Windows Update Setup Files folder. Then look for the ie6setup.exe file. I doubt that will happen, but just in case...
C:\Windows Update Setup Files\ie6setup.exe
If this doesn't fix the problem, please email me. Include the Windows version you're using and the Exact version of Internet Explorer. (See Internet Explorer/Help menu /About Internet Explorer for the information).
I hope this helps!
Dwight