Mike, you know, since avast! version 4.7.807 the mail scanner module (“Internet Mail” provider) has been significantly changed to improve the overall user experience, especially in case of slow connections (dial-up). Namely, most of (if not all) the “Timeout expired” related problems should be gone by now.

You can increase both your avast! timeout: Internet Mail provider > Customize > Advanced (tab)
and your email account timeout (generally, right click the email account, choose properties and the Advanced tab of settings).
See pictures here: http://forum.avast.com/index.php?topic=17460.msg148474#msg148474

In a dial-up, 600 seconds could be normal with large files…