Each Store in Exchange 2000 has two database files associated with it. One is the normal *.edb database that holds most of the content, the second is a *.stm streaming database. This contains all incoming internet messages. The reason for this is that it prevents exchange from having to convert multiple mail formats in order to place them in the normal database, instead it only references messages in the stream file from the users mailbox thus reducing overhead. The two are intrinsically linked, so if one is unavailable, as in your case, the store won't mount.
That hopefully answers your first question, as for the second, I'm not familiar with Norton Corporate. I'm assuming it has detected a virus in an incoming internet message (not surprising) but if it's designed to block such messages it seems odd that it would block acces to the whole stream database. If it is designed to work with Exchange I would suggest that you check you configuration documentation for Norton Corporate to make sure its treating that file as a database and not just a normal file. If it's not an email scanner, then you will need to create some sort of exclusion to prevent it from scanning your databases. In that case, just un-quanrantine the file and you should be able to remount the affected store.
