Hi and welcome to the forum.
When starting up Outlook you should see the Avast Plug-in 'splash screen'.
I had an issue similar, a few months ago and did a "Repair" of Avast which resolved the issue at that time.
TO repair Avast: Windows, Add Remove programs, select 'Avast!' click the Change/Remove button from the pop-up window, scroll down to Repair, click next and follow instructions/reboot.
Did that help?
Edit:I actually use Outlook 2003 but I think (not absolutely sure)the same process should work for Outlook 2000