We might need some more details to help you.
Does the installer runs every time you open Outlook or just the first time you open Outlook 2k7 after installing Symantec Antivirus?
If it runs just once, you can consider it normal.
If it runs every time you open Outllok (and just outlook), the problem might be related to Office installation…
Does this particular Office installation have been upgraded from a previous version (2003)? If this is the case might want to consider removing all references to the previous office installation. Please go to Control Panel and remove all Office 2003 components. After this, make sure you clean every reference of it with Windows Installer Cleanup Utility.
If may assumptions are not correct, please correct me.
In any case, please give us some feedback.