With systemworks being a suite of sorts (sorry I have no experience of it at all), I don't know if you should uninstall that and then do a custom install of it and ensure the AV element isn't installed.
Sorry I have only limited suggestions as I don't use NAV or Outpost, just general experience, but I know NAV would integrate with Outlook in a similar way that avast's Outpost/Exchange plug-in does so this may cause the issue.
You could check in Outpost and see if there is still a NAV plug-in still enabled. I don't know what these other plug-ins do or if they might also have any effect, perhaps you could try disabling them temporarily and see if that helps.
Or try an uninstall of Outlook, boot and reinstall. This is obviously not something to take lightly you would need to back it up so as not to lose contacts, emails, etc.