You should always backup your important data just to be on the safe side. If your important files are on a separate partition e.g. D:, E:, F:, etc. it should be intact as long as you don't delete or reformat the partition from the custom options of the installation wizard. What version of Windows are you talking about? Basically you should be able to use a repair Windows/installation option, if there's something wrong with the currently installed OS.