>>>What is your and clients SQL Server versions? Backups are not backward compatible.
>>>
>>>>A client sent me a database (.bak) file. I'm trying to restore it. I'm getting
>>>
>>>>TITLE: Microsoft SQL Server Management Studio
>>>>------------------------------
>>>>
>>>>An exception occurred while executing a Transact-SQL statement or batch. (Microsoft.SqlServer.ConnectionInfo)
>>>>
>>>>------------------------------
>>>>ADDITIONAL INFORMATION:
>>>>
>>>>The media family on device 'C:\Program Files (x86)\Microsoft SQL Server\MSSQL.1\MSSQL\Backup\ER_VT_backup_2011_05_10_020001_3850000.bak' is incorrectly formed. SQL Server cannot process this media family.
>>>>RESTORE HEADERONLY is terminating abnormally. (Microsoft SQL Server, Error: 3241)
>>>>
>>>>For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=09.00.5000&EvtSrc=MSSQLServer&EvtID=3241&LinkId=20476
>>>>
>>There will be a different error message if this is a version compatibility problem.
>
>
>What if the backup is created with compression and you don't have Enterprise version?

That's a possibility.