For a more rigorous scan, then Regseeker does a good job without destroying your computer. Though you should always make a restore point, and a backup, before using a more aggresive registry cleaner of this sort http://www.hoverdesk.net/freeware.htm
Watch out for so called free reg cleaners. Some have a habit of installing the very software they are supposed to clean.... After all why are they free? Someone is going to make some cash somewhere along the line...
There is an awful lot of free and opensource software that is extremely good. I provide a lot of IT support on these forums free, and just about everything I recommend has been tested and checked by me first.