Well if you have rights to use it you must have purchased it at some point in time. Which means that a serial had to of been issued. If it was a pre-installed OS then check the side or back of the system for the serial. If you bought it with Vista but own a copy of XP you should still have a serial for it to allow for activation. Unless there is something in there i am missing about how you are licensed to use the software (XP).