When you signed up for OneCare with a credit card online, the agreement you accepted was that the service was auto-renewed annually. There is no way to not agree with this if you purchase online. If you subscribe with a retail key from inside a package, you can't be charged for renewal, so you are asked for payment information or another key at renewal time.
If you wish to cancel the service and obtain a refund, contact OneCare support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
If you are in North America, you can call this number (not to be used for technical support) to cancel:
1-866-672-4551
-steve
Microsoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator
