Richard is quite right when he states that registry cleaners are not needed or should be used with Windows 7. However, CCleaner is one that is recommended by many people here for its comprehensivity and ease of use. Even so, you should take precautions and make a backup of the registry before making any changes to it. That way, if you do run into problems, you can restore the registry to its original state.
You also say that you are having trouble with your system, and that you believe it to be something to do with your registry. What issues are you having with it? They may, or may not, be something to do with the registry.