Use System Restore to Undo Changes if Problems Occur
Windows XP Professional makes it easier to resolve problems if they occur in your system. You can use System Restore to remove any system changes that were made since the last time you remember your computer working correctly. System Restore does not affect your personal data files (such as Microsoft Word documents, browsing history, drawings, favorites, or e–mail) so you won’t lose changes made to these files. Windows XP creates “restore points” every day, as well as at the time of significant system events (such as when an application or driver is installed). You can also create and name your own restore points at any time. Creating a restore point can be useful any time you anticipate making changes to your computer that are risky or might make your computer unstable. If something goes wrong, you select a restore point and Windows XP undoes any system changes made since that time.…
http://www.microsoft.com/windowsxp/pro/using/howto/gethelp/systemrestore.asp