You may have a lot of files that have been deleted , but still on your system - try running wisecare 365 (Free program) to clear these, and do a defrag - more efficient than windows.There is also options to defrag your registry and other tools. There is a paid version, however I find the free version does all I need.
Check you do not have more than 2 backups stored on your system-if so remove them.
Check for unwanted programs and remove ( use windows or wisecare 365), which also checks the registry and remove links from there as well
If you need to do a 'Clean' windows install, go to 'settings'' then 'recovery', and follow the options, BE WARNED- this will remove most of your installed programs, so ensure you have the installion software to re-install them