If you see the avast! splash-screen when starting Outlook, then yes, you are using the Outlook plugin (and it's not disabled).
I'm afraid I don't have any advice regarding the described behavior... it's really strange. Do you have any anti-spam program installed? Or anything else, that might be in the mail-processing chain?
Are you able to simulate the problem? I mean, you wrote that it mostly happens with e-mails that have already been answered... if you can simulate it (most of the time at least, e.g. by sending specific message to yourself), you can disable avast! to see if it helps... even though I don't think so.