Para detalhar os comandos que você está vendo:
bcdedit /export C:\BCD_Backup
** Export the current BCD (Boot Configuration Data) to C:\BCD_BackupC:
** Change current working drive to the C: drivecd boot
** change the current working directory to the hidden "boot" directory on the current drive (C:)attrib bcd -s -h -r
** remove the SYSTEM, HIDDEN and READ-ONLY attributes from the file "bcd" (where boot configuration data is stored)ren c:\boot\bcd bcd.old
** rename the "bcd" file to "bcd.old" (backing it up)bootrec /RebuildBcd
** actually rebuild the "bcd" file, from scratch
Conforme observado nesta ajuda da Microsoft página
The /RebuildBcd option scans all disks for installations that are compatible with Windows Vista or Windows 7. Additionally, this option lets you select the installations that you want to add to the BCD store. Use this option when you must completely rebuild the BCD.
Basicamente, você está fazendo backup, removendo os dados de configuração de inicialização e, em seguida, forçando uma reconstrução completa dele.