System Restore is a utility (for Windows XP, Windows Me, and Windows Vista) which allows you to restore your data back to a previous date.

How to use System Restore (Windows XP): Click Start -> All Programs -> Accessories -> System Tools -> click System Restore, make sure that Restore my computer to an earlier time is selected and click the Next button. Choose a restore point (the dates ou can choose from are in bold), and then continue with the Wizzard.

System Restore will undo any changes made to the sytem but your files and all programs installed will remain intact.