It is possible that the computer retailer who set up the PC for you already activated OneCare for you, but didn't complete it successfully. You have two choices - go back to the retailer who installed OneCare or contact OneCare support to determine the LiveID for the key in your retail package to make sure that it isn't a duplicate. If it is a duplicate, you'll need to get another retail package from the retailer in exchange for yours.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
Note also that if you had a 15 day trial period a week after you got the PC, it sounds like you may have reinstalled OneCare as that is the period of time you get after an install from CD before activation is required. If the PC was obtained a week ago, you should have about 7 days left for activation.
If you didn't reinstall OneCare, you may have a system date/time problem.
And, finally, I wonder if you created a new User ID in Windows and OneCare was setup under a different account and wants you to activate with the LiveID for that subscription...
-steve