I've been able to activate windows xp about 4 times. Then when it refused to activate the 5th time, I called the phone number. I explained to the lady what my situation was. I build my own computers and over the years have gone through several changes on my one system. She gave me a special code and it seems to have "reactivated" my activations.
They just do the activation thing to keep people honest. If you talk to somebody personally, you can work something out.
Eventually you probably wont be able to activate it anymore. Maybe Longhorn will be out by then.
Sooner or later microsoft will have enough products requiring activation that we'll have an easier way to do it. Maybe something like Half-Life 2.
I dont particularily care for any of it. But the software companies think its the best way to protect their interests.