System Restore lets you undo recent changes to your system by taking you back to point before the changes were made using something called a restore point. It will not change or cause you to lose any data you have saved or files you have created.
To use it right click on the Start Button and then click System. Then click on "System protection." The in the System Properties window click on "System Restore" and select the date of the restore point you want to go back to.