If you have OneCare on another computer, open OneCare, click Help/About... and the LiveID is shown as the subscription ID.
You can change the password for your LiveID at http://account.live.com
If you don't see the subscription ID, support can tell you the LiveID if you can provide the retail product key and/or credit card and contact information.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve