Restore or Reinstall Windows

If you do not have a good backup, a time may come when it is necessary to perform a clean install of your operating system.  As much as possible, back up any files and settings you want to keep so that you can restore them later. 

When reinstalling the OS, it will be necessary to also reinstall your programs.  Before you start, make sure you have the installation discs, product keys, or setup files.

These Microsoft articles have more information: