So it seems, the Win7 Loader by Daz (v2.2.1) is not working after you downgrade from Win10 to Win7, even if you had a working Win7 before.
Error message "The certificate wasn´t successfully installed"
How to fix:
get MBR Regenerator v4.5
Check "Regenerate all activation system" and press "Start Regeneration System"
it will take some minutes, it may look like nothing happens, but you'll just have to wait.
then restart and try the loader again (run as admin)
Tip: do not install the Windows Updates KB971033 & KB3035583
if already existing, uninstall them (check Google for how-to) and ignore them