CCleaner

It's simply the safest registry cleaner and optimizer I've used. It's free too, and you can easily back up your registry in case you want to restore anything. It's very simple to use, and suitable for amateurs and pros. I recommended it over anything else in the market, 1) because it's free, 2) and it's safe.

Publisher's Description:

CCleaner is a freeware system optimization and privacy tool. It removes unused files from your system - allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history. But the best part is that it's fast (normally taking less than a second to run) and contains NO Spyware or Adware! :)

Links:

Official Site: CCleaner
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