Your Yahoo! Toolbar might have been disabled as part of the Internet Explorer upgrade process. You'll most likely need to download Yahoo! Toolbar again:

To make sure your browser is ready:

1. In Internet Explorer, click on the "Tools" menu and choose "Internet Options".
2. Click the "Advanced" tab.
3. In the "Browsing" section, make sure "Enable third-party browser extensions" is selected.
4. Close all IE windows and restart.

Now reinstall Yahoo! Toolbar at http://toolbar.yahoo.com.