I never knew Avast baught out Piriform. Sad because I actually liked their software for a time. My info is current circa mid 2017, so take that as it may be.
If you're trying to combat the inevitable accumulation after using your computer for a couple years, and have the know-how, spend a couple hours reinstalling windows from scratch. then press "h", move along, and enjoy your clean slate.
If not, grab the portable CCleaner version from the official site and you can avoid most of that bloatware khomus was talking about shipping with the installer. CCleaner helps you wave goodbye to shit like Farmville,manage startup items, event viewer logs you'll likely never need, internet history you should've deleted months ago, cache, recents, thumbnails, registry entries and data from long-since deleted apps. Not quite the same thing obviously, it's like slapping a band-aid on a festering wound, but still a low-tech maintenance solution.
It's the difference between wiping up and pulling out your entire deep cleaning setup. You do the former as much as possible and the latter every once and a while, or after fucking up and making a huge mess -> installing Avast or something. The sad truth is that after checking background and startup apps, there's only so much you can do about a system entering the inevitable phase of decline.
Registry cleaning only works up to a point, and it will never under any circumstances make your system any faster. It only mattered in the days of I think it was windows 2000, when there was a limit on the amount of data that could be held (somewhere around 20% of the paged pool). And even then people just increased it and went on their merry way. I just avoid them like the plague. Most work by searching predefined paths for files and folders that no longer exist, which isn't very reliable for obvious reasons. So in short, registry cleaning is overrated and unnecessary.