Hello, Jennylht.
If you don't recall the LiveID you used for your subscription, you can contact support who can provide that information to you using your Key and/or contact information. Use the instructions to open a ticket as a trial user to start an email case.
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 for your subscription.
-steve