I've done that with Windows XP. You just install normally, don't select any of the repair options. It pops up finally with a "Hey! You already have Windows installed. Do you want to repair it?" Only then do you say yes and it repairs your existing installation without changing any settings.
Of course, Murphy's Law dictates that Vista and 7 probably upgraded that feature to one that completely nukes your system. If that's what you're running, better check with someone with first-hand experience before venturing forth.
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