Normally if you uninstall and then reinstall, as long as the hardware fingerprint hasn't changed, your activation is still good from last time and it is not counted as a new activation. You can tell because it'll start up right away without a "retrieving key" message displaying on the screen.
I don't know if the hardware fingerprint changes because you move from XP to Vista. What you say about the fingerprint not changing because you're using the same hardware is logical, but unfortunately the activation system itself is not always logical.
I wouldn't worry about it too much. Our intention is not to prevent paying customers from accessing their games, and we're going to make sure that no valid customer ever has to repurchase a game from us simply because they've used up their activations.
EDIT: I just looked up your order and I will send you a PM about your activations, if you're curious.