I realize that this thread is now over three years old, but perhaps my recent "discovery" will assist somebody needing to reinstall their current Windows installation with the same version. If you just need to reinstall the Windows version that you have due to some files getting lost or corrupted, you can perform an upgrade to the same OS that is currently installed and not lose your user files. This is the same process as upgrading from Windows Vista to Windows 7, for instance, but you are "upgrading" to the same OS version. Note, this does not work, for instance, to upgrade from Home version to Professional version or any other version change. You can only use this procedure to reinstall the exact same version as currently installed and you cannot change your installation key in doing this.
edit: corrected spelling error