Could you clarify what you are trying to do? Are you trying to purchase a new subscription for OneCare from the payment screen arrived at during the activation process from within OneCare? What error, if any, are you encountering and at what point?
If you are entering the payment information for the credit card details, are you perhaps entering any characters that are not letters or numbers? Dashes, commas, periods, apostrophes, etc, will cause problems with payment submission.
-steve