You do not provide credit card information to use the trial. What you did was to click on the link to subscribe and have activated a subscription, effectively ending your trial and immediately starting a 12 month subscription.
Unfortunately, now that OneCare has been installed and activated on your PC, simply contacting support, canceling the subscription and reinstalling the trial will only net you a 15 day trial period.
Note that you clicked through several confirmation screens during the purchase process that indicated that you were purchasing a subscription that would begin immediately upon purchase.
-steve