When you wish to activate OneCare and subscribe, you need to sign in with a LiveID (like the one you used to sign into the forum here). You then provide a retail product key or payment information (credit card) to begin your subscription. Your LiveID used to activate is you subscription ID and will be used for reinstalls, installs to a 2nd and 3rd PC while your subscription is active, and to manage your account. You will also need this ID to renew the subscription.
If you need help in finding out what the ID was that you used to install OneCare and you have just reinstalled, you'll need to contact support -
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
If you have an active subscription and OneCare is activated and running, your subscription ID can be found under Help/About in the main OneCare interface.
-steve
