Hi Ian,
You could try this:
- Reapply MDAC 2.8
http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=6c050fe3-c795-4b7d-b037-185d0506396c
- Run a repair of Backup Exec installation
- Since it does concern the Master database..do you have SQL Server installed on the media server? If so, make sure you solve the SQL issue as the Master database MDF and/or LDF files might be corrupted
Hope this helps :)
Sam