I moved your post from the Off Topic folder where I found it, to the Install and Activate topic folder.
If you are installing OneCare to a second PC, you need to use the same LiveID as on your original install/activation as the key can only be used one.
If you have never activated a subscription, your key is a duplicate and you need to exchange your copy at the retailer from where it was purchased.
-steve
Microsoft MVP Windows Live / Windows Live OneCare Forum Moderator