If you know the LiveID, but forgot the password, get it reset at http://account.live.com
If you don't recall the LiveID for your subscription and OneCare is no longer installed and activated, contact support. Select the option in the flow that you are a trial user and provide the details of your problem. With your contact information, credit card, and/or retail key, support can advise you what LiveID your subscription is under.
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 all of your LiveIDs until you find the OneCare subscription listed.
-steve