How do you put your original product key in your computer without reformatting your computer?

Each and every product have their own product key. for example if u have installed Windows XP that's not original has its own product key, later u have got the original but that particular product must be installed for its own product key

i suggest u go for installation without formatting the computer. the newly installed will just overlap on the older one without any disturbance for the present program files etc, if you have any errors or virus threats go for fresh installation ok..gud luc