Well, you can just remove the NG part if you want. Use Programs and features and use Change menu in avast! uninstaller. It will only remove the (problematic) NG part and the rest will remain where it is.
Sorry, but avast force NG if you install the free version and there's absolutely NO WAY to uninstall NG, and if you're prepared to wait about a month (in extreme scenarios), then that's the time it'll take for NG to finish.
My PC has been on continuously for 3 weeks and NG front end is still using 35% CPU,with core temps ranging from 120F to 145F, at one time it got to 150F and the system shut down due to overheating, all because NG, and I'm still waiting for it to finish, it it takes more than a month, I'll be switching to free AVG.