You are getting a renewal notice from OneCare via email? When you sign in at http://billing.microsoft.com with your subscription Liveid, what is the status of your subscription? I suspect what you are reporting is that you reinstalled OneCare and it now wants you to subscribe. If that is the case, you need to subscribe - or activate - the copy of OneCare to "hook it up" to your subscription, which you created in March. To do this, you need to use the same LiveID you used to create the subscription in March.
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
