If you have installed onecare on another computer, and activated it using your subscription you can check your Live ID by going to onecare
click on help
about Windows Live OneCare
it will give you your subscription ID
But in case you have not, and you have no idea or totally forgot your Live ID. Please contact support.
How to reach support (FAQ) -
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2