Why do you think so?
To see if avast! is scanning emails, check one of these points:

  1. Is the avast icon (the special one of email scanning) be shown in the system tray?

  2. Is your email header with the lines X-Antivirus: avast! (VPS XXXXXX-X, XX/XX/2007), Inbound message and X-Antivirus-Status: Clean? (Right click the message, choose Properties and Details)

  3. Are clean notes added at the bottom of the email? (Internet Mail provider settings)

[b]You can test the security of your email system here: http://www.gfi.com/emailsecuritytest/[/b]

Make sure the avast plugin is not disabled into MS Outlook:

Outlook 2007 > Tools > Trust center > Add-inns > Manage Disabled items.
Outlook 2003 > Help > About > Disabled items.
Outlook 2000 > Tools menu > Options > Other > Advanced Options > Add-In Manager.

As reported a lot of times, the message “The provider is waiting on the subsystem to start” will be shown there until you open MS Outlook.
The plugin will be enabled automatically after that.

If so, send the files called unpXXXX to vlk (at) avast.com
They may contain more information about the problem (maybe a link to this thread).

I suppose avgas and not the antivirus.
Did you have any other antivirus in this computer in the past?