EasyBCD takes no part in the boot process. It's purely an application for managing the contents of the BCD.
All of the booting is done by MS bootmgr, which then chains MS winload.
During Installation of a newer version of Windows alongside older versions, the dual-booting is handled automatically by MS setup. (You can subsequently use EasyBCD on the running OS to tidy up the BCD cosmetically, but it's not necessary to use it at all to get an additional OS in your multi-boot if you're happy to live with things like "New version of Windows" as your boot menu entry)
The default action of setup is to use the pre-existing BCD from the original Windows installation (on the "active" partition) and add an entry for the new OS in there.
Since an older version of Windows cannot boot a newer, it will also replace the old version of bootmgr with the later (bigger) version from the newest OS
failed during the boot process
Do you mean during one of the many reboots that needs to take place during setup ?
It's necessary to make sure that DVD (or whatever installation medium you are using) is set in the BIOS to be higher priority than the HDD on which the system is being installed.
Otherwise, at the first reboot, instead of returning to the DVD for the next stage of the install, the system will attempt to boot from your half finished install with obvious results.
