Sometimes a Windows installation will become unbootable. You might turn on your computer one day and see nothing but a mostly blank screen with a confusing error message.
When this happens your files are usually still on the hard drive, but you just can't boot into Windows. The Knoppix live CD is a great solution to this problem becuase it allows you to boot up your computer from a CD and have access to all of your files even if Windows won't boot.
Once you have booted the computer into Knoppix, you can copy the data on your Windows hard drive onto an external storage device like a USB thumb drive or external hard drive, or even a digital camera. (See the note at the bottom of this post.)