Resurrect SQL Server Agent after repair an instance

For Spanish version visit this link Resucitar Agente SQL Server después de reparar una instancia
After a failed update from SQL Server 2005 to SQL Server 2008 R2 SP1, I decided to use the Repair option of the installer, the SQL Server resource was repaired correctly, but the SQL Server Agent couldn’t start.
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Rollback process for a failed SQL Server Cluster Instance migration

For Spanish version visit this link Proceso de marcha atrás para una migración fallida de Instancia en Cluster SQL Server
Take a practical case in which have to migrate a production SQL Server instance from Windows Server 2003 Cluster to Windows Server 2008 Cluster.
One of the first steps for a migration is shut down the SQL Cluster service and delete the name of the SQL Cluster in the Active Directory, because if we not doing this, the Windows Server 2008 Cluster that we create will find a duplicate name when try to create the SQL Cluster, here we find a problem if in the middle of the migration had to rollback, because the SQL Server resource in the Windows Server 2003 Cluster not will start.
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