Windows XP includes a useful feature called System Restore. This utility saves "snapshots" of your computer's configuration and settings, allowing you to revert back to a previous state if something goes wrong.
According to Microsoft, System Restore isn't capable of modifying, deleting or restoring your personal files that are stored on the hard drive. So while you don't have to worry about them being erased as you revert back to a previous state, be aware that System Restore cannot restore files that were accidentally deleted.
Open up the System Restore window. This can be done by clicking "Start," "All Programs," "Accessories," "System Tools" and "System Restore." Alternatively, you can open the menu by clicking "Start," "Help and Support" then selecting "Undo Changes to Your Computer With System Restore" under the "Pick a Task" section.
Once the System Restore window has appeared, select "Restore My Computer to an Earlier Time" and click "Next." You will be presented with a calendar of the current month. Each day that is in bold contains at least one restore point. Select any day that you know the computer was functioning properly. It's always best to choose the most recent known working date, to reduce the probability of errors. Click on "System Checkpoint" on the right side, and press "Next" to continue.
You will be asked to confirm that you want to restore to that checkpoint. Once you press "Next," the restoration will begin, and any open programs will be closed. Save any unsaved work before continuing, or it could possibly be lost. It may take 30 minutes or more for the restoration to finish. Once it has completed, Windows will restart using the older settings, and you will be returned to the desktop.