Avast's free anti-virus product is excellent (I use it myself) but it does occasionally try to tell you that you need another (paid-for) product to get rid of all the nasty stuff it claims to have found. The reality is that most (all?) of what it finds can safely be left in place anyway.
If you really want to use a free 'clean-up' program then the free version of CCleaner can do the job for you
https://www.ccleaner.com/
as can the equally good Glary Utilities:
https://www.glarysoft.com/
(There are dozens more programs which claim to do a similar job but many of them can cause more problems than they fix. Those are the only two products which I'd use myself).