From past experience, I've had to jump through hoops to get my legitimately purchased XP Pro reactivated when I reinstall. With Vista and crap like this, I will never purchase another MS OS. There is absolutely nothing besides game playing you can do with Windows that can't be done with free software.
That depends on the circumstances of the reinstall.
I've found many Dell computers that used a CD key for the installation that is different from the key on the sticky label. That causes problems on reinstalls, but it is a Dell problem, not a Microsoft problem.