Install OneCare on the PC that you wish to add(after removing other security software) from
http://onecare.live.com and after it is installed, open OneCare and click on the link to subscribe. Enter the LiveID that you used to purchase the subscription with and it will become part of your Circle/subscription.
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator