Reinstall or add a computer from http://onecare.live.com - member accounts - add a computer. Activate with your LiveID for your subscription.
Also, if this is a new install and you do not have a subscription already, you cannot activate a new subscription any longer.
See this post: http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/0000c782-ed1b-4ed3-8afb-87989bb9638c
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