The .BAK files from SQL server are in Microsoft Tape Format (MTF) ref: http://www.fpns.net/willy/msbackup.htm
The bak file will probably contain the LDF and MDF files that SQL server uses to store the database.
You will need to use SQL server to extract these. SQL Server Express is free and will do the job.
So, install SQL Server 2008 Express edition, use sqlcmd -S \SQLExpress (whilst logged in as administrator)
then issue the following command.
restore filelistonly from disk='c:\temp\mydbName-2009-09-29-v10.bak'; GO
This will list the contents of the backup - what you need is the first fields that tell you the logical names - one will be the actual database and the other the log file.
RESTORE DATABASE mydbName FROM disk='c:\temp\mydbName-2009-09-29-v10.bak' WITH MOVE 'mydbName' TO 'c:\temp\mydbName_data.mdf', MOVE 'mydbName_log' TO 'c:\temp\mydbName_data.ldf'; GO
At this point you have extracted the database - then install Microsoft's "Sql Web Data Administrator". together with this export tool and you will have an SQL script that contains the database.