Since when do you have to buy a copy of windows for every machine you have in the house? I reformatted my computer yesterday and used the family’s winXP key when I installed windows again. I updated it today, rebooted and not it says it’s not a valid copy of windows!
Also, the key has been reported stolen! How?! 
We’ve always used the same windows cd on our computers, maybe we missed something, but it has worked for several years now!