How to Transfer old Server License To New Server License For Windows Server

Ok, my problem is simply this..I just got a new server that i want to add my existing license to from my old server. But when i try doing that it says it is already in use. So i deactivated the server and the problem still persists. Now i have no idea what to do can somebody please assist me in this problem.
By the way I run a windows server 2003 R2..