Thanks for answering. You may be on to something because I didn't actually uninstall NAV because it wasn't the stand-alone version of NAV, it's part of Norton System Works. I don't have the NSW CD anymore so I didn't have any way of uninstalling NSW and then reinstalling it by leaving NAV out of the installation.
What I did was turned off NAV's auto-protect and disabled everything in NAV's settings and then I went to msconfig and disabled all instances of Norton so nothing of Norton would run on startup. I figured if NAV was turned off completely and not running at all, it shouldn't interfere with Avast.
When I installed Avast it didn't recognize any previous AV installations or give any warnings of the presence of another AV. The Avast installation went without a hitch.
As for MMM, I didn't import any accounts into it, I created the accounts manually. And a while later when I switched to Avast, I manually changed the server and username info in MMM so Avast would recognize it, e.g. 127.0.0.1 and myusername#mail.myisp.com.
Since MMM and Avast worked for you, I don't know why it's not for me. It could be NAV interference for sure but since NAV is completely disabled and there's no Norton running in the background I don't see how it could be especially since Avast works fine in Thunderbird. If NAV somehow is interfering in MMM it seems like it would in Thunderbird too. I don't know, I'm stumped.