I will try to explain again, better:
Avast has got 2 add-ins that run inside Outlook.
1st is called "Avast 5" inside Outlook, on disk it is ashOutXt.dll - under Outlook 2007 it can be found in Tools > Trust centre > Add-ins. Select "Exchange Client Extensions", click "Go".
2nd is called "Avast! Antuspam" inside Outlook, on disk it is asOutExt.dll - under Outlook 2007 it can be found in Tools > Trust centre > Add-ins. Select "COM Add-ins", click "Go".
HTH