Oh no it wont be cleaned completely, but yes everything that should be cleaned out is meant to be cleaned out. Sometimes though, there can be sticky files that get hitched, kind of like with an interrupted install, but they are not corrupted, just sticky I suppose. I get this a lot with install and uninstall of tv tuner cards. But I have to say not with avast. I just wonder if something else is 'sticking' a file or two, something that we would not expect to do such a thing.
The only other thing is perhaps install in Safe Mode. I think you can do this - even though avast will not run in Safe Mode. You just boot back into Normal Mode, and have to manually start avast I guess. I have never tried it. perhaps you cannot install in Safe Mode.
Good luck anyway.