Microsoft Knowledge Base Article 883256 contains the following summary and introduction:

This article describes how to change the status of a Microsoft Internet Explorer add-on and how to update an add-on by using the Manage Add-ons feature . This article lists some troubleshooting tips to help you resolve problems that you might have with add-ons . It also lists some troubleshooting tips to help you resolve problems that you might have with the Manage Add-ons feature itself . The article describes how to manage add-ons by using registry entries and Group Policy . Administrators can let users manage certain add-ons but reserve management of other add-ons for the administrator . Or, administrators can manage all add-ons . These topics are covered in specific sections that discuss the following policy settings:

Deny all add-ons unless specifically allowed in the Add-on List
Add-on List

INTRODUCTION

Microsoft Internet Explorer add-ons are software components. When you visit a Web site, Internet Explorer add-ons may be downloaded automatically, or you may be prompted to download an add-on. For some add-ons, you might specifically visit a Web site to obtain the latest add-on for Internet Explorer. These components may be third-party ActiveX controls that extend browser functionality or that provide special user interface elements in Internet Explorer. You can control add-ons by using the Manage Add-ons feature. This article describes how to use and how to troubleshoot this feature.

To make sure that the Manage Add-Ons dialog box is displaying as many add-ons as possible, update the Add-on Manager. To update the Add-on Manager, visit the following Microsoft Web site:

http://www.microsoft.com/downloads/details.aspx?FamilyID=D788C59E-B116-4D38-B00C-FF1D529106C8&displaylang=en

http://support.microsoft.com/kb/888240