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. Scroll down to see your Subscription ID. That is the LiveID to sign in with.
Unfortunately, you've also encountered a bug in 2.0 that will be fixed in the future. Once you enter a LiveID in OneCare for use in the trial version to enable multi-PC functionality, you cannot change that when you wish to activate a subscription. Your only solution at this point is to uninstall OneCare and then reinstall which will allow you to enter the correct LiveID.
-steve
