How can I clean up my computer without erasing my computer and get the same result?

I'm on Windows 10 Pro. My computer is very, very slow...

In the past, I would back up all my files, erase the hard drive and start all over. 

I want to avoid that if I can. I've got Avast as both protection and as a service, as far as going through and cleaning up unnecessary files and I've done all that, but the performance of my computer is still slow. 

And by, "slow," I mean applications sometimes saying, "Not responding" etc.

Are there any other options besides completely erasing my hard drive and starting over that will do the same kind of thing?