Your computers registry is a database of important settings that dictate how certain aspects of your operating system and software run. However, computer are so complex, that your registry often fails to keep up with the current state of your computer perfectly. This happens as a normal part of computer use, but over time it can severely slow down your computing experience.
To fix this you need a registry cleaner. Registry cleaners are programs that run automated scans of your registry, searching for inconsistencies and mistakes. Registry cleaners then can correct these problems, making backups if you want, and help restore your computer to it's older, quicker self.
One of the best registry cleaners in the industry is Piriform's CCleaner. Not only is CCleaner an excellent registry cleaner, it also has several other functions that make it an invaluable tool.
CCleaner can do an automated clean up of your computer, clearing such things as your browsing history, cookies, temp files, recycling bin, windows log files, autocomplete form history, search autocomplete, and more. This takes care of all the period junk that piles up in your system every few months or so.
The big kahuna, CCleaners Registry Cleaning function is truly top-notch. This is best used if you have used your computer for months without a registry cleaning, or simply if you have just made a lot of changes on your computer (such as installing or uninstalling several programs, or removing spyware).
The tools function is an advanced program and services uninstaller. It can uninstall programs, like you can under the control panel, but more quickly, and more then one at a time. It can also disable startup programs (which you can normally do from run->"msconfig"). Both these are slightly more user-friendly than there windows counterparts, so they are very useful.
Overall CCleaner is a top-notch, if not the best free registry cleaner available for windows.