If you have an active subscription, which is what was created when you activated on the first computer, you need to use the same LiveID on the subsequent activations. You can't use the same key twice. If you are arriving at a screen to purchase or provide a token, you are likely not using the same LiveID as the subscription ID.
On the first PC, open OneCare and click Help and then About to see your Subscription ID. That is the LiveID to sign in with.
And see this post - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2776450&SiteID=2 - if you are unable to change the LiveID in the activation sign on screen.
-steve