Every computer that uses Microsoft Windows has a "registry" - which is basically just a place on the computer where a bunch of settings are stored for how Windows will work. So what is a "registry cleaner?" Well, after you've had your computer long enough, and put enough stuff on it, you can get a lot of clutter in the registry. Also, a lot of malicious programs (viruses, or advertising programs that install themselves without your permission) will put stuff into your registry without you knowing. The reason is that this makes it much, much harder to get rid of the malicious program. Registry Cleaners Guide will help you eliminating the junk, but leaving in everything important .