Sorry, haven't noticed you are using MS Outlook... anyway, solution shouldn't be too far from that. Another good example, how people should post brief configurations of their systems, list of most important security programs they have installed etc.
It can immediatelly give a picture about the problem and possible solution.
So, we know your mail client is MS Outlook, what's your OS, which version of avast! (Home Edition or Pro), which version and build, what virus database version etc. ?