Backup / Restore to different server - "Media family is incorrectly formed"

I am very new to using MS SQL Server.  I offered to help a friend out not knowing what I was getting into.

I have a .bak database, I downloaded and installed MS SQL Server Express 2005 - 9.00.1399.06 and I using Management Studios.

I right click on Databases, select Restore Database - select FROM Device: and I get this error.

TITLE: Microsoft SQL Server Management Studio Express
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An exception occurred while executing a Transact-SQL statement or batch. (Microsoft.SqlServer.Express.ConnectionInfo)

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ADDITIONAL INFORMATION:

The media family on device 'F:\afmfirearmsdb.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=09.00.1399&EvtSrc=MSSQLServer&EvtID=3241&LinkId=20476

And when I run the following command that I found on these forums

RESTORE VERIFYONLY FROM DISK='f:\afmfirearmsdb.bak' 

I get this error:

Msg 3241, Level 16, State 7, Line 1
The media family on device 'f:\afmfirearmsdb.bak' is incorrectly formed. SQL Server cannot process this media family.
Msg 3013, Level 16, State 1, Line 1
VERIFY DATABASE is terminating abnormally.

I am very much a noob when it comes to working with this stuff, so all the help would be awesome.

Thanks