As Tech has advised you if you are using the Web interface to send/receive Yahoo and Hotmail then your email is not scanned by the Internet Mail Provider. The “clean” notes and X-Antivirus header are only inserted when you are using an email client (like Outlook Express, Eudora, Thunderbird etc) and receiving your mail on standard POP/SMTP accounts. This is no difference here whether you are using avast or AVG.

While GMail is also accessible via the Web interface it can be used with an email client but then it required by GMail to use secure connections to the POP & SMTP servers of GMail. The whole intention of using secure connections is that they cannot be intercepted (by anyone or any antivirus program) so neither avast or AVG will insert the messages in GMail unless some other special steps are taken.