Avast Anti-virus Program CAn it Scan Web-based e-mail Prog..

I recently downloaded AVAST 4.1.357 a free anti-virus program which seems
very good but I have the following questions:.

1)Is this program capable of scanning web-based e-mail such as
Yahoo and others? If so how might that be done?
2) If it does not scan incoming and outgoing mail,, does AVAST
scan attachments on these we-based e-mails?
3) In general, do most anti-virus programs only scan web-based
e-mail attachments and not the e-mail itself?

If anyone has information on these
questions , I would be very appreciative .

Thank you,
,