i think you can keep it underwraps if you update or register your windows.
Unless you got an OEM, then its only for 1 computer. I think for Retail, you can install it on a certain amount of computers, but you are only suppose to install it into one.
OEM is cheaper, but sometimes when you upgrade something, then the key might not work, and you would have to call MS, while with Retail, you wouldnt need to call.
At least thats what i hear.