From Registry Recovery Help:
When you first purchased your computer, you were probably very impressed with how fast it was and how smoothly it operated. Over time, you may have noticed that your computer boots more slowly than it did when new or that programs quit unexpectedly or that errors have begun to pop up. All computers that run on a Microsoft Windows operating system (Windows 95 to Vista) run into similar issues. Usually this is due to errors or unneeded information that accumulates within a directory known as the Windows registry. Changes are effected within the registry whenever you uninstall existing software, install new software, delete files, use the chat program or create a new user. As you go on using your computer, new information is continually being added to the registry of your computer. Various unused keys, incorrect entries, invalid keys or adware and spyware keys can cause problems within the registry. It is important to carry out regular cleaning and maintenance of the computer in order to keep the computer running smoothly and efficiently.