If you enable Restrict Anonymous on an Exchange 2000 box (which you should do) and you are running Backup Exec 8.6 you will start seeing these messages in your job logs:

     Unable to attach to \\MOM\Microsoft Information Store\First Storage Group.
     The item was not found.

     ^ ^ ^ ^ ^
     Unable to attach to \\MOM\Microsoft Information Store\First Storage Group.
     ^ ^ ^ ^ ^

Previous versions of Backup EXEC are not compatible with the Restrict Anonymous setting but you can make a quick and easy change to the registy as described here to fix the problem and continue restricting those anonymous connections.