On the payment page there should be an option to use a "Pre-Paid PIN or Token" as well as credit card payment. If you do not see this option I suggest uninstalling One Care, then install the One Care trial -
http://onecare.live.com/standard/en-us/default.htm
After installing the trial, open One Care, select "Subscribe" and when you reach the payment page you should see the option to use a "Pre-Paid PIN". If you are still unable to subscribe using the product key from your CD I suggest contacting support. How to reach support - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2