Win32:Malware-gen; File: Invoice_20130912.zip/Invoice_20130912.exe

Every couple of minutes, I get an avast Mail shield pop-up: “Infection detected! avast! Mail shield has detected a threat. Infection: Win32:Malware-gen. URL: (null). File: Invoice_20130912.zip/Invoice_20130912.exe.”

avast! does not delete or do anything with this file, and I keep getting the messages whenever Outlook (Office for Mac) is connected to my Hosted Exchange account.

I have run a Full System Scan and it cannot be found. According to Finder, the file does not exist anywhere on my network.

I have the latest 10.8.04 Mac operating system.

What should I do, if anything?