Your using Outlook 2003?
Cool, then just disable the avast Internet mail service and that's it. Outlook is handled separately, by the Outlook/Exchange plugin.
Go to Start -> avast Antivirus -> Mail Protection Wizard, and choose Remove protection from all my accounts.
BTW do you see the avast splash screen when starting Outlook? If not, make sure avast is not listed in the Disabled Items field, accessible from within Outlook About box...
Thanks
Vlk