If you did not activate a retail copy and have lost the key (meaning you never used it and have since misplaced the retail box with the key), there is nothing that can be done for you. If you *did* activate or purchased online during activation, you can activate a reinstall by using the same LiveID that you used to activate the subscription.
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.
-steve
