TechRepublic.com: "A long time ago, in a land forgotten by time (before Microsoft Windows 95), Windows computers and their programs had .ini (configuration) files that stored information. These files determined how a program ran, its environment, and a host of other things. Then the Windows Registry came onto the scene, with each key in the registry being similar to a bracketed heading in the old .ini file and values similar to entries under the .ini headings. However, registry keys can have nested subkeys with string or binary data that .ini files don’t support. Does the registry periodically need cleaning? Let’s take a look."
This is a good article on why you should not worry about cleaning your registry with those tools that promise to optimize it. I have used a few of those 'registry cleaning tools', and they have done nothing to improve performance of my computer or save disk space.
These program have disabled my applications and computer. So I really don't recommend any application that promises to make my computer run faster by optimizing the registry.