OK with Vista and Windows Mail avast should be scanning your email.
To check this you can look at the notification area when email is being scanned there will be a new icon in there, see image.
The avast 'a' icon should also rotate, check the Internet Mail provider and the Last scanned: and Scanned count: fields should be continually updated.
You can also check the emails headers as there should be a couple of entries for the avast scan.
If you don't know about SSL then it is unlikely that you are using it, though if your ISP gave you a setup program to save you having to manually setup the email accounts, then you may not be aware of that (unfortunately I can't tell you where to look I don't use Windows Mail). However, if your email was using SSL connections on the standard ports the sending of emails would fail as currently avast can't handle the SSL connection.
So if there are no errors and any of the above indicators should it being scanned, no problem, dial-up shouldn't be an issue, that is what I use.