Windows applications consist of many individual components. Information about these components is stored in the ActiveX and COM areas of the registry which are available to applications installed on your computer.
The sounds that are played when certain application or system events occur (for example, when an error message is shown, or when you shut down Windows) are stored in sound files on your computer.
The installation folders for some programs are saved in the registry. This lets these programs be started through the Run dialog without entering the complete application path.
Many applications (for example, virus scanner programs or service programs) start up automatically when Windows does. When you remove such applications from your computer, orphaned entries can be left behind in the Autostart region of the registry.
File types link one or more file extensions, like .TXT or .DOC, to various kinds of information. For example, they specify the icon that should be used in Windows Explorer to represent this type of file, or the program that should be used to open them.
The registry contains references to files which were intended for use by more than one application. If the specified files are moved or deleted, the references may become invalid.
The registry contains information about which folder contains the help files for a given application. If the specified help files have been moved or deleted, this part of the registry may contain incorrect entries.
The fonts that are installed in Windows are registered in a special part of the registry. Errors in this part of the registry should be corrected in any case, because Windows will unsuccessfully attempt to load the corresponding fonts every time the system is started.
In the Control Panel, the "Programs and Features" entry in Vista ("Add and Remove Programs" in older versions of Windows) lists the programs that you can remove from your system. This list is generated from a special part of the registry, which can also sometimes contain errors.
The registry contains references to system extensions such as audio and video codecs or Internet Explorer add-ons that enhance the functionality of media player programs and Internet Explorer. References to these system extensions can become invalid if the extensions are moved or deleted.
The Windows Firewall stores information about which applications can accept incoming network connections. When such applications are moved or deleted, the corresponding information can also be removed.
Every program installed on your system can store program-specific information, such as data or program paths, in the registry. These entries may not always be correct and are sometimes left behind by a failed or manual uninstall process.
Database drivers and data sources can be installed or registered in the registry. They are sometimes needed by programs even if your computer isn't used primarily as a database server. These entries may not always be correct and are sometimes left behind by a failed or manual uninstall process.