Yeesh.. alright, this scares me a little bit.
The system uses the old win2000 machine name insted of the new domain name which was created when migrating to the SBS2003 machine.
I'm not quite sure if your server is correctly named and on the domain.
Have you ever ran a dcdiag on the server to see if there were any DNS, naming, or any other errors? Maybe a netdiag too (I can't remember if that was omitted in 2008 or not, so you might not have it).
What I would like to know is if the installation package for the SBS should be different from the installation packages on the other two servers hosting ISA 2006 and SQL 2005?
In case, what should this package contain?
No, the package is the same, it's just the "server" installation that's supposed to be used. Really, the only difference between the small business suite for avast, and the standard server versions are pricing and licensing. The SBS suite doesn't come with anything different.
The avast proxy would probably be included in the web shield. Maybe try turning that off on one of the clients and see if that helps the outlook issue or not.
Can you tell if the Avast Exchange plugin is running on the server?
Frankly, I don't even know where to start here. It's really unclear to me where the problem begins. Whether it's an authentication problem because the server is named incorrectly, or if it's a plugin (exchange or otherwise) that's not functioning like it's supposed to, or maybe a DNS error, pointing to the incorrect server.
Does everything else on this network function properly (besides the avast/outlook issue)?
I'm even more confused than earlier.