I don't use XP, but if I am understanding what you are wanting to do (I use Win200 Pro), try right clicking on my computer, click on properties, go to the Network Identification tab, the click on the Network ID button and go into Network Identification Wizard. I think this is where this is accomplished but I am not positive because I always do this during the initial installation of the O/S therefore I have never had to do this.
Good luck.