I have already disabled / uninstalled NG through add/remove programs and rebooted, but I was unable to recover any disk space. System restore was taking up 9GB of my small OS SSD. NG.
I have now set the system restore disk space at 5% which gave me a message that some system restore points will be deleted, I let it delete them and recovered 6-7 GB already. Let me see after a reboot whether avast! without NG will again set the system restore disk space reservation to 50% or leave it at 5%.