If you already subscribed and are trying to activate a reinstall or on a second PC, you need to use the same LiveID used for the original subscription activation.
If you don't recall the LiveID you used for your subscription, you will need to contact support who can provide that information to you using your Key and/or contact information.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
Alternatively, sign in at http://billing.microsoft.com with email addresses that you have set up as LiveIDs and one will show your subscription listed - that would the LiveID to use to activate the reinstall.
If you are trying to subscribe and cannot log in with your LiveID, if your password exceeds 16 characters, shorten it at http://account.live.com. If you are pasting your password into the form, type it in. You may also want to delete cookies, clear temporary Internet files and close IE before trying activation once again.
-steve
