Just by way of elucidation on CCleaner: It is most often used to get rid of clutter (temporary files, cookies, histories, Recent Document list, empties the rubbish bin etc). I run it about once a week and it usually gets rid of between 100Mb and 200Mb of crap that I don't need.
It also has a (separate) tool that checks Registry Integrity - before you run this, it offers to back up the registry. I don't run this as often but it usually picks up an appreciable number of inconsistencies (it calls them "Issues") and will then fix them for you.
Thirdly, it shows you what gets cranked up during Startup and allows you to Disable them (and you can "Re-Enable" them if you have any problems).
A very useful piece of software - and it's Free
(although they don't mind if you bung them a couple of bucks as a donation!).