Get the job scripts out of a MSDB Backup
Perhap simplest would be to do the following:


Install a new instance of SQL Server.
Shut down the SQL Server, delete the msdb data and log files.
Copy the old msdb data and log files to the same location as the files you deleted.
Bring up the new instance of SQL Server and script out the jobs.  (You can use SSMS Object Explorer Details to select all jobs and create one massive script, or do them one at a time.)
Uninstall the new instance. 


There are also scripts, including this powershell script, but they use methods that talk to msdb:
http://www.johnsansom.com/script-sql-server-agent-jobs-using-powershell/