1. In Outlook, navigate to the Tools --> Trust Center menu
2. In the Trust Center dialog select Add-ins on top left, then select Exchange Client Extensions in the "Manage" drop-down box and click "Go"
3. In the Add-in Manager locate and uncheck the unwanted add-in (in this case "Symantec Antivirus 10.1") and then click "Ok"
4. Restart Outlook - the add-in will no longer start and it will be gone from the Add-in Manager
These steps will work to remove/uninstall/disable any custom third-party Add-ins in Outlook 2007 - Symantec Antivirus Add-in, SAP Calendar add-in, XobniRdo Add-in, et cetera.