TuneUp Utilities is also worth considering
I absolutely agree with Leo. Not only does Tuneup Utilities clean your registry, it also has an application which Defragments the registry as well.
So let's say you've installed a Windows Service Pack and your registry has become so bloated, that it has to read from a second page before your system boots. Tuneup Utilities Registry Defrag will condense your registry back to a single page.
I'm not too sure of any other application that can do this type of aggressive maintenance.
One word of advice. Stay away from the "One-Click" Maintenance, instead run the apps independently.
Hope this help;
Cp