Are you getting an error when providing the credit card information?
Check the information that you are entering for name, address, etc. on the payment form and make sure that no characters other than letters or numbers are included. No dashes, punctuation, etc.
Also, make sure that your profile at http://billing.microsoft.com and http://account.live.com correctly identify your country.
If you are still unable to provide payment information online, you can either opt to purchase a retail copy of OneCare and use the PIN/Key to activate the subscription or contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve