Oh, so you are talking about the Outlook or Internet Mail provider, i.e. about e-mails? Well, that's something else then (I thought you meant scanning your hard disks).
If you are using full MS Outlook (not Express), you can configure it directly in the configuration of the resident provider - Outlook plugin (the second page - what to do if avast finds an infected message).
If you are using Outlook Express, or any other e-mail client, i.e. you are protected by the Internet Mail provider, then I'm afraid it cannot be set directly in the resident protection configuration window. It can be set only in the Enhanced User Interface (i.e. in the Pro version) - edit the resident task, and modify the Virus action for the Internet Mail provider (remove "Interactive selection" and set "Delete" instead).