There are a number of fairly reliable and free solutions that will read your Yahoo mail (via http/browser type access) and convert it to POP3 mail to be delivered to a mail client (Outlook, Outlook Express, Eudora, Thunderbird etc.) and allow your mail to be scanned by Avast.
A few of the free solutions that come to mind are YPops, MrPostman, FreePops. Of these YPops is specific to Yahoo, Mr Postman will provide access to Yahoo, Hotmail and quite a few other mail services. FreePops will provide access to Yahoo, Hotmail and a large range of other webmail systems. If you are a Thunderbird user there are (relatively) new plugins to support Yahoo and Hotmail conversion to POP3 too.
They are all very easy to use with Avast (whatever you may read here to the contrary). The key factor to remember (if you are an XP user) and use any of these solutions to convert Yahoo to POP3 is to uncheck the "ignore local communication" box in the Avast Internet mail scanner. Then you will have POP3 mail delivered from Yahoo and also validated by Avast.
My Yahoo and Hotmail accounts are being delivered as POP3 to my Thunderbird client many times a day and always scanned by Avast.