Users reported experiencing odd browser behavior after installing the most recent security update which has been identified as being caused by a registry key.

Problem:
Some reported opening a browser window that promptly hangs IE, others have reported opening links that render blank. There are also reports of multiple windows opening when initiating a browser session.

When:
This can happen if a user has ever attempted to run IE7 Beta1 in an unsupported side-by-side configuration with a version of IE6 or after a failed IE7 installation.

Solution:

To address this issue on a machine running IE 6 SP1 with our most recent security update, locate and delete this entire key from the registry of the affected machine: HKEY_CLASSES_ROOT\CLSID\{c90250f3-4d7d-4991-9b69-a5c5bc1c2ae6}. If you are running IE7 Beta1 in a side-by-side scenario with another version of IE, this is not a supported scenario. Please uninstall and reinstall IE7 Beta1 in the recommended manner.

Source: IEBlog