Reloading windows on a drive with windows on it? USALLY(but don't hold me to this...) XP will load the new copy to another folder and you can selected which to boot from via a boot menu. However, if it's the same version of windows, it might not allow it. Typically, though, I don't try this.
When you go to reinstallw indows on the same partition as a previously loaded xp, it will igve you the option ot install it to a new folder, like TOO said, but it iwll also gove you the option to delete windows folder, and reinstall windows through that way.