Go to Control Panel and run Add/Remove Programs. Select Office 2003 and click Change. Under Maintenance Mode Options, choose Add or Remove Features. On the next screen, check the box for "Choose advanced customization of appliications." On the next screen, you should see the CDO option under Outlook and be able to select it. Click Update to finish the installation.
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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers