When you re-install Windows 7 and decide to create an "image" of your squeaky clean new OS, you will go to "Control Panel", "Backup and Restore" and then create a "System Image".
Also on that page is "Create a System Repair Disc", this is kept in a safe place just in case Windows 7 has completely died on you and Backup and Restore won't work. You dig this disc out from your safe place and your computer will boot from it and then ask for your Image disc, it will then replace your broken W7 with the image.