Adding to Jim's comment, the OneCare team is aware that activation is a pain point for many, especially when adding another PC or reinstalling when you already have a subscription. They are working hard to improve this experience.
I'll also add that an IP address as an identifier for your subscription is pretty useless as most ISPs provide a dynamic IP address, meaning that it can change at any time. Dial-up users will get a new IP address every time they connect. Further, if you are connected via DSL or Cable, the IP address facing the world is actually the IP address of your primary device connected to the modem, usually a router. Your PC obtains an IP address from the router, and this IP is internal to your LAN and is likely to be the same LAN IP address of literally thousands of other users with the same router.
-steve