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!

either way it's against MS's licensing policy and that would have been obvious if you'd tried to activate online.