First try backing it up with NTBACKUP. NTBACKUP you will have to search the log for skippped or in use error. This means there is something wrong on the OS side.
Backup Exec - Enable restrict anonymous in registry and restat the backup exec services and then try the backup
http://support.veritas.com/docs/238618
If that fails try disabling antivirus during the backup.
Side Note: 8.5 is no longer supported by Symantec.
Lyon