PC World recommends Defragment your Registry to boost your PC's speed.
Cleaning your Registry with a tool such as Eusing Free Registry Cleaner will go a long way toward speeding up your PC --- but it won't go all the way. Even when you clean out the Registry, it may still be bloated. That's because over time, the Registry becomes defragmented, particularly when you add and remove Registry entries. This free, simple tool defragments the Registry, and gains you a bit of hard disk space. More important, though, is that it speeds up your PC's access to the Registry by compacting it -- and that means a speedier PC. The program is free, it's simple to use, and it speeds up your PC, what else could you ask for?
--Preston Gralla
http://www.pcworld.com/downloads/file/fid,65031-order,1-page,1-c,systemresourcestuneup/description.html
There are some very good registry cleaners out there, one being Registry Smoker http://www.xp-smoker.com/regsmoker.html , and it only cleans the registry.
There are programs like Diskeeper that do it all as far as Defrag is concerned.
Most people have problems with registry programs, any program really, because they don't read the instructions and take the necessary precautions.
Searching the Internet, i.e. Google, Yahoo, will give you helpful and valuable information, start your research here: http://www.google.com/search?q=registry%20Defrag&sourceid=groowe&ie=utf-8&oe=utf-8
Make it a great day!