There are 2 different kinds of windows keys. OEM keys; which are one time use and on activation get tied to a motherboard and cant be activated to a different motherboard*; and Fully Licensed keys, Fully licensed keys can be used any one computer at a time.
*This is the way it is supposed to be, but if you call the support hotline the indian guy behind the other line has no problem re-activating your key for you, for free aswell.