Yep, started by me. Check out the thread here:
http://forum.avast.com/index.php?topic=41775.0I forget what the registry key it is that has exchange check to see if there is an A/V scanner plugin active before mounting the stores, but if you google the error that you see in the event viewer I'm sure you'll find it.
Basically, something is happening on the Avast side that is preventing the plugin from loading properly and scanning the exchange database. When that happens, Exchange immediately dismounts the stores and won't let them mount again.
You can either follow the steps in the thread above, or google the error that you're getting to find the registry key that you have to change in order to prevent exchange from looking for AV protection, and have it mount the stores anyway (obviously I don't recommend this).
All I can say is that SBS2008 can be a pain in the rear to get setup (and I'm about to setup an SBS2011 system, so I'm not looking forward to it!), but if you read the instructions that a few fellow users contributed in the above thread, you should be able to get it running correctly.
We're all eagerly awaiting the new version 6 for business that we can only hope will come very soon. If so, SBS 2008/2011 should be a piece of cake. We won't really know until it comes though.
Good luck and come back if you have anymore questions!