When you activate OneCare after a reinstall or on the second or 3rd PC, you must use the same LiveID as your subscription. It sounds like you may actually have paid for a second subscription. It will not ask for payment or a product key if it finds a paid up and active subscription already associated with that LiveID.
You should contact support to make sure that you have not purchased a second subscription.
As for the expired trial, you need to sign in with the same LiveID as on your PC that is already running an activated copy of OneCare - see Help/About in OneCare to see your LiveID.
How to reach support - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2 If it fails to validate your subscription, select the option that you are using a trial or beta copy and you can proceed to email support without validation once you've signed in.
-steve