If you are activating a new subscription and have never used the retail key before, you will either need to exchange your copy at the store from which you purchased it or contact OneCare support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
If you are reinstalling or activating on a second or 3rd PC, you must use the same LiveID that you subscribed with as the key cannot be used again.
-steve