But the short version is this article -> http://www.flexbeta.net/main/articles.php?action=show&showarticle=3
There is a Microsoft article but this one is simpler to follow. You don't need to reinstall the OS.
I have a computer with XP office O.S. that can not be authenicated. I have an original disc of XP home registered in my name, can I install the home edition over the office XP without problems?