I assume that you mean that your credit card payment form is being rejected after you have signed in? Or are you unable to sign in with your existing LiveID?
Sign in with your LiveID at http://account.live.com
Make sure that your contact information is correct, particularly your country.
Make sure you are located in a country where OneCare is available for sale -
http://onecare.live.com/standard/en-us/faq.htm
Clear your cookies and temporary Internet files in IE.
Close IE
Start activation from within OneCare
Sign in with your LiveID
In the payment/contact form make sure that you are not using punctuation or non-alpha numeric characters like dashes, apostrophes, hyphens, and commas.
If you are still unable to proceed with activation, please contact support:
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve