Verify the backup file using the following script:
@filename = 'C:\Program Files\Microsoft SQL Server\MSSQL\BACKUP\northwind.bak'
Msg 61900, Level 19, State 1, Line 0
Failed to load XML data into document object. The integrity of the backup file could not be verified.
Backup file may be corrupted due to possible hardware or network problems.
If the backup file is corrupted, use an uncorrupted backup file to restore database.