I have a copy of Windows 7 Professional 64 Bit thats already been installed on a computer. However I want to use the CD to install it on another computer. Is it possible to do this and then purchase a valid Windows 7 Activation key from Microsoft? Or do i have to buy a new Windows 7 CD+Key together? Would be much quicker for me to buy the Activation key over phone or internet! 
Thanks!

Thanks!