To be sure:
Is the avast icon (the special one of email scanning) be shown in the system tray?
Is your email header with the lines X-Antivirus: avast! (VPS 0XXX-X, XX/XX/2004), Inbound message and X-Antivirus-Status: Clean? (Right click the message, choose Properties and Details)
Are clean notes added at the bottom of the email? (Internet Mail provider settings)