Not too sure about Vista but XP you have the option to repair without wiping the data , it copies your data and apps over and then reinstalls windows and restores the links (I've done it in the past) You loose your system restore points but it will start to make new ones right away. All you do is insert the disk and reboot and you'll get to the options to install windows, there should be an option for repair or reinstall, use repair.

I'll hazard a bet that vista will have a similar option, if your not too sure what your doing then I would suggest having a look on google or MS's website just to make sure.

It's prudent to back up your data anyway just in case (regardless of the OS, vista, XP, OSX, linux) as things can always go wrong.