Read:
http://www.licenturion.com/xp/fully-licensed-wpa.txt
It uses a one-way hash to detect if certain hardware changes, with no way
for MS to know exactly what hardware that is.
Gee, you click a button. Big deal. First, if the hardware hash never
changes, you don't have to wory (ie, you can reinstall all day long and
never run out of activations.) Second, it allows you to change several
items of hardware before having to reactivate. Third, if you do have to
reactivate, the count gets reset every 6 months, and you they allow several
activations before having to call in. Fourth, even if you manage to exceed
all these things, you need only call the MS call center and tell them why
you are legally re-activating so often, and they'll reset you.
That review is so out there, it's hard to believe. As i said, XP won't
make you call MS unless the hardware changes.