Backing up the Windows Registry is not as important as backing up your data, but it is still a good idea. If Windows starts acting seriously weird, restoring the Registry to a point when it was healthy could save a lot of grief.
Windows provides two methods for backing up your Registry. The first is Windows' built-in, semiautomated system backup tool, which protects a number of important files besides the Registry. Windows is supposed to create a restore point every day or so automatically, but do not trust that to happen. To create your own restore point in XP, select Start*All Programs*Accessories*System Tools*System Restore. Select Create a restore point, click Next, and follow the wizard.
In XP, you can restore a previously saved point via Start*All Programs*Accessories*System Tools*System Restore and follow the prompts.
The second method is to use Windows' Registry editor. To launch the editor, select Start*Run, type regedit, and press ENTER key. To make a backup, select File*Export. For the Export range, select All, and otherwise select and click as needed. This method creates a .reg file; all you need to do to restore is to double-click it and confirm that you want to change the Registry.
But restoring the entire Registry from this backup does not always work. Even when it does, it will not restore any new keys that were created after you made the backup. If a restore fails, reboot into Save Mode and try again. This improves the chances that a restore will work - but no guarantees.