How to backup the Windows XP Registry:    Registry Editor is an advanced tool for viewing and changing settings in your system registry, which contains information about how your computer runs. Windows stores its configuration information in a database (the registry) that is organized in a tree format. Almost all the settings are stored in the registry, for example, when you resize an application window, the x,y axis points [window pos] are stored in the registry so that the settings are retained next time you open the application. This is just an example, there are much more stored in the registry right from your user account names and passwords [if configured to store in the registry].

Although Registry Editor enables you to inspect and modify the registry, normally you do not need to do so, and making incorrect changes can break your system. Therefore, it's highly advisable to backup the registry before editing/deleting any key. Registry Editing is required only if there is a specific need (if the User interface does not provide an option, for a specific setting, or if you want to script the changes across different systems)